Skill Leveling Tips (Oblivion)

Armorer
Repair all your own equipment, always. Bring 20-30 repair hammers with you into the field, for this purpose. Repair the equipment and weapons of your fallen foes too, for extra gain. Use disintegrate weapon or disintegrate armor to increase the total wear on their equipment. This is most effective on foes with better, heavier armor, since it takes a larger number of attempts to fully repair.

Athletics
Find a body of water with a suitable wall and use auto-run to keep swimming into the wall. Go for lunch. Works with running into a corner too, but slower.

Blade
use conjuration to summon a skeleton and bash it from behind (will level you in conjuration as well, however). Most of the time they go down before they turn on you. The safe way is to cast, attack 2-3 times and recast, dispelling the first skeleton.

Block
See Heavy Armor, below. And hold down the Block-key.

Blunt
see Blade

Heavy Armor
Once you've gained some semi-decent armor, find a rat and let it hit you over and over again. Optionally level Restoration at the same time by using a tiny healing spell to counter the damage.

Alchemy
Alchemy is trivial to level up, if you collect all the ingrediest you see as you're doing other things. Occasioanlly visit the nearest mages guild to borrow their equipment, and just start making potions. Since it's only the number of potions that is important rather than their usefulness, buying food from tavern owners is also an option for getting ingredients.

Alteration
Cast the smallest Shield spell you have over and over again on yourself. Spell level does not affect gains, so cast as cheap a spell as possible. Even faster is casting Open Very Easy Lock on any lock of a harder grade (ie at least an Easy lock) -- the lock won't open, but the skill will increase, fast. This also works on doors that don't lock like the large ones in Imperial City.

Conjuration
Only Bound Dagger is useful for leveling Conjuration to 25, as you can put the dagger away (default 'f' on the PC), thereby cancelling the spell so you can cast it again.

Destruction
After gaining access to the Magic University, create a 3-point, 1-second damage-on-self spell and cast it over and over again. Optionally raise Restoration at the same time by countering the damage with a tiny healing spell.

Illusion
Cast a tiny Light spell over and over.

Mysticism
Cast Minor Detect Life over and over.

Restoration
See Destruction or Heavy Armor, above.

Acrobatics
Place something on the jump key. Go for lunch. Place something on the run key and run into a corner of a wall. Go to bed.

Light Armor
See Heavy Armor, above.

Marksman
Get an iron bow and as many iron arrows as you can carry. Find a troll, then climb up onto a steep cliff or rock, where it is unable to follow, and start shooting. The painted trolls seem to be especially tough, with plenty of rocks in the enviornment. Between the low damage of iron bows and arrows and the trolls natural regeneration, the only limit is the number of arrows you can carry.

Mercantile
Find an item that both buys and sells for 1 gold (if such an item exists, unconfirmed). Buy and sell said item over and over and over again. Barring that, sell all items one at a time, even arrows and other large stacks. Remember to move the haggle bar (any setting that's not all the way to the left works for skill gain).

Security
Continuously fail to pick a lock (quick double clicks do it). Very expensive, however.

Sneak
Continually sneak around Rufio in the Inn of Ill Omen and you can level up your Sneak skills indefinatley. If you kill Rufio as part of the Dark Brotherhood initiation quest you can no longer take advantage of this.

Speechcraft
There is no easy way, but keep talking to random townsfolk including guards and playing the mini-game and you can improve it. After some speechcraft increase, you can play the game against the same people again (but usually only 1-2 rounds). In practice prioritizing the small wedge for the hated reaction (it will rotate clockwise as you make choices) and one of the larger wedges for the best reaction is a good starting strategy.