When using a Skaven army, the one thing you must remember is to take advantage of your high movement rate. Skaven are not a defensive army. With a Movement of 5, and Verminlords and packs of rat-ogres and giant rats with a Movement of 6, you can rush at your opponent. Close the distance quickly to minimize losses due to artillery and missile weapons. Above all, outmanuever your opponent. Use your speed to flank your enemy and to deny them of deffendable terrain like walls and hedges. If your Grey Seer gets the Death Frenzy spell, you can make a large unit speed into battle as well as turn it into a mincing-machine.
The first point a Skaven general must remember is the importance of ranks. Skaven are notoriously cowardly when not in a large group. With a standard Leadership of 5 and the general having a leadership of 7, the Skaven need all the help they can get. The rank bonus from multiple ranks confer a bonus to their leadership, up to +3, which gives the general's unit a respectable Leadership 10. A good tactic is to have one or two huge blocks of rat-men. When I say huge I am talking about at least 30 Skaven. Clanrats and Plague Monks work best for this. I personally prefer Clanrats, as a large group of Plague Monks can be hard on the wallet. My usuall main blocks consist of 31 Clanrats and a character. After you have your main blocks of Skaven, add Warpfire Throwers, Poison Wind Globadiers, and Assassins to give it extra punch.
Now you have a couple of large blocks of chittering rat-men just itching for a fight. The first thing you have to incorporate into your battle plan is how to protect your flanks so that you don't loose that all-important rank bonus. There are numerous ways to accomplish this. First, you can take rat swarms and small packs of giant rats or rat ogres to protect your flanks. Another way is to use your main blocks to support one another. Set them up an inch apart from each other and advance across the battlefield in this manner with a rat swarm protecting one side and a pack of giant rats on the other. Using the board edge is another good way of protecting a flank. Run a large block of Skaven down the edge of the battlefield so no enemy unit can charge one flank. Remember to use terrain to your advantage as well. When advancing stay close to wooded areas and other sections of difficult terrain where those pesky high-flyers can't swoop down into.
High-flying monsters are one of the hardest problems for a Skaven army to overcome. Personally, I don't like taking large monsters or units of harpies because they don't seem very Skavenish. Skaven are underground creatures who only forray to the surface to raid. I can't see them being used to using flying creatures of their own, but any good general makes due with what he has. There are numerous magic items to counter flying creatures charging your flanks and taking away your rank bonus.
My favorite is the Warpstorm Scroll. It causes any model flying high to suffer D6 S6 hits and forces them off of the board. The drawback is that it is only a one use thing so make sure you get it off and hopefully you'll at least soften up those pesky flyers.
An item good for protecting your Warlord or Grey Seer from powerful flyers like Daemon Princes and High Elf Lords on Griffons is the Talisman of Ravensdark. With it, flying creatures must roll a 6 to hit.
The Orb of Thunder can be useful as well. It prevents flying creatures from flying high. Creatures already flying high cannot descend. The effect remains in play and you get three uses. Of course it can be dispelled and that is the real drawback. Hold it until your last action in your magic phase and try to get your opponent to use up those dispells before evoking its power. I personally like to use it to keep high flyers out of the battle.
A pricy magic banner at 100 points, but incredibly useful against shooty armies like Wood Elves and Dark Elves, is the Storm Banner. High flyers are forced to land in the center of the battlefield and are moved 3D6 inches according to the Scatter Dice. No troops may fly and shooting ranges are halved. these effects last until a 6 is rolles at the start of either player's turn. Another drawback is that your core units won't be able to carry it as they can't take a magic banner (unless you prefer Plague Monks that is), or you'll need to use the Army Standard Bearer to carry it.
The Scarecrow Banner is a cheaper magic banner that I use more often. It causes terror in flying creatures and adds +D6 combat result when fighting flying creatures. I usually give this to a unit of Stormvermin as they tend to be picked out by big monsters. Either that, or using the Army Standard Bearer, I stick it in my warlords unit.
If you can't tell by now, I HATE FLYING CREATURES!
Personally, I like to choose my troops and then pick magic items tailored for my opponent. If this is not possible, I choose magic items that can be used in many situations, such as wards, weapons, and armour. You can never be too sure of your Warlord or Seer's survival. Any item that helps Leadership, like the Crown of Command, is also desirable. A particularly nasty thing to do is give your main block of warriors the Dread Banner which causes fear. You almost always outnumber your opponent and with your Warlord fighting and a couple of assasssins, if you can afford them, you definately tip the scale in your favor. One problem you may run into is that enemy calvary speeds across the field, thus eliminating your tactical advantage of speed. Take care of this with the Pipes of Doom which cause D6 S4 hits on calvary units. The unit won't be able to charge the next turn either, giving you plenty of time to close the distance and tear the tin-cans appart. So let the Piper work for the rats for once. The Skaven have a number of magic items unique to their race. If your opponent is unfamiliar with Skaven, these can give you quite the psychological edge.
Praise be to the Horned Rat, for he has heaped upon us many blessings in the form of powerful spells! I think Skaven magic is one of the most powerful in the game, especially the Grey Seer spells. To that extent, I always include a Grey Seer in my army, even if it means cutting down on auxilary troops or powerful magic items. In smaller battles of 1000 or 1500 points, you can dominate the magic phase because your opponent seldom takes a level 4 mage as they are so costly. Grey Seers are costly for Skaven as well, but the cheaper core units allow you to take one.
Now, with the help of my Grey Seer Hrist, we will discuss the tactical advantage of certain Skaven spells.
Pestilent Breath uses the cone-shaped template from Warhammer Magic. Placing the pointed end at your sorcerer's base, any models touched by the template suffer one wound on a roll of 4+. No armour save allowed except for magic armour. This spell is excellent for taking out well armoured or high Toughness opponents. The only drawback is it's slightly short range. Also, don't hit your own troops as they are just as sucseptible to it as your enemies.
Warp Lightning is another spell good for use against heavily armoured troops. It has a range of 24" and inflicts D6 S5 hits on the unit, each wound causing one wound with no armour save possible, even for magic armour. Neek Neek!
Cracks Call is especially deadly to the less agile races. A crack shoots forth and travels 18" in a straight line. Any model in its path must roll under its Initiative on a D6 or fall to the their death. A 6 always fails and chariots, war machines, and the like must roll a 5 or 6. Buildings suffer D6 damage points. This spell is excellent against units of Dwarfs, Orcs, Giants, and the like. Their low Initiative make for quick kills. It is also useful for taking out troops fortified within a building.
Plague is a good spell to use against massed ranks of low Toughness troops like Empire halberdiers, large units of Elven Spearmen, and huge mobs of Gobbos. The range is 18" but only affects one model at first. The target suffers 2D6 minus its Toughness wounds. If this kills the victim you can move the plague onto the nearest model within 4" who suffers 2D6 minus its Toughness wounds. This continues until a victim survives or there are no other targets within 4".