The method given in the D&D 4e DMG isn't bad: use a card deck.
I use index cards:
- 16 skirmishers
- 8 brutes
- 8 soldiers
- 6 artillery
- 4 controllers
- 2 lurkers
- 4 minions
- 1 solo.
So you make one card for each, just go through the deck.
Next, shuffle them up, and then run through it again to assign a level that ranges on each card from party level -2 to party level +5. with the majority of the numbers being equal to party level.
Shuffle them up again, turn to the back of the monster book of your choice (or monster builder or what-have-you) and pick out monsters that fit each card. For example "Brute level 4"- you just look what brutes fit. Mark it down.
You could be totally random about this or more structuyred- for example, doing an orc or fey themed deck. See examples below.
Finally, building encounters: figure out the encounter budget and draw the first few cards to fill up that encounter. Skirmishers count as two monsters, solo's count as 1, minions count as 4. A lurker gets added in after you do the entire encounter. Keep drawing until it fills the XP budget up.
Final step: try to figure out how this makes an encounter. if it makes sense, you have an encounter. If it doesn't? Just draw more cards.
Good: You draw a gnome arcanist, a pseudo dragon, and a group of wisp-wraiths (minions): great encounter. The gnome and the dragon are friends, adventuring together. The wisp wraiths are firefly spirits in a jar the gnome is carrying around. He just smashes the jar in round 1.
Needs work: You draw an orc raider, an orc brute, a halfling slinger, and several spiders. hmm. You could drop the spiders, and redraw. Maybe the halfling is a particularly charismatic bad guy, and the orcs are his gang.
As you start to increase the encounter area, adding new monster cards can be used to create a more themed area. For example- you could create a less random deck that is used just for underdark areas or a sylvan woods.