When I was first reading through the 4E monster manual I had to do a double take at the minion entries. What the heck was a 1 HP monster? It took me a while to wrap my AD&D mind around having low HP monsters that were literally fodder. Slowly I began to get an understanding the role of minions. It wasn't necessarily a new idea. I particularly always thought Mutants and Masterminds had a clever idea using mooks (basically one shot minions) for their superhero D20 game.
For a while though, I struggled to use them. They were very squishy and seemed to drop too fast. I had some difficulty using them as an effective screen for other important creature types in fights. I could never get any reasonable positioning with them around the PCs. In short, I never felt my group had a challenge when I used a fair amount of minions so I started to shy away from commonly using them in encounters (just a filler of 2-3 to round out the XP budget). After a while though, my perspective changed.
I use minions quite frequently now. They are a perfect way to add a lot of bodies in a room, and not completely overwhelm the characters. Yes, they do drop like flies. However, that does go well with my group. I think the players enjoy getting to feel powerful hacking through 4 to 8 guys before getting that evil wizard. Minions do work, but I found I needed to rethink how I used them.
Use their role - Check the DMG (pg. 54-55) and read up on monster roles. Starting with MM2, WotC has now started including this information in their monster stats. When you are thinking about setting up and running encounters, take this information into consideration. Most minions fall within the skirmisher role followed by brutes, but a few fall into other roles. This does have a small impact on how you run them, so keep it in mind. Also be sure to use your minions to support the other monster types in a fight. Positioning is important not to mention the following point...
Use Aid Another and Combat Advantage - Most PCs forget about aid another (PHB pg. 287), a DM shouldn't. Get 3 minions adjacent to a player. Have 2 make a melee basic attack at AC 10. Odds are with combat advantage that 3rd minion that can now add a whopping +6 to his attack roll. Aid another is a great way to add some teeth to a minion's bite. And if you've got your minions supporting a brute in combat, watch out. With aid another that brute will likely get a big bonus to his heavy-hitting attacks.
Use a lot of them - The 4 minions per standard creature in the heroic tier is a great yardstick, but I don't consider it a hard rule. I usually will add another 1-2 to the entire encounter if I'm using minions. The group will make short work of them (and they should!) but you can hamper the group's movement with a threat of attacks of opportunity, and generally providing a shield of bodies to the more important monster types. When using minions, be sure to not quibble with their numbers.
Throw waves of minions at the players - I used to have just about all my monsters out on the map the start of a fight. Sure I would have terrain and cover strewn about, but the players ended up having a good grasp of the battlefield right at the start. A simplistic view could be seen right depicting a smaller room. The group enters the south part of the room with a mix of monsters like artillery, brutes, and minions (M) ready to face off against them. In this situation, the players see all the opposition and can quickly plan out maneuvers and coordinated attacks. A worse case scenario, the group's controller aces a initiative roll and splatters most the minions in one shot (note the room is a bit smaller than what I would use for a standard 5 player group).
I stopped doing this when I have a fair amount of minions in the mix. I now usually keep a least half in reserve for the first turn. I get the players out into the room and see the general position of everyone, then I commit the rest of the baddies. One thing I have really strived for now however is to have multiple pathways to the same room (illustrated below). Here the player's have to worry about additional creatures coming in from different directions. I can push the minions to pile through one entrance, or spill into the room from 2 directions. Even if the players turtle up down in the south west corner (in the below example), they are cluttered together for effective blasting by the artillery/controller types.
This really keeps my group on thinking on their feet. Definitely start to consider using your minions in waves. Avoid committing everything at the first initiative roll (lurkers work well keeping them back also). I also heartily suggest creating encounters where the players have to worry about multiple routes of attack. If the players have to face off to other directions, it ramps up the excitement and makes the combat more dynamic.
These are a few tips I have for using minions. I'd love to hear other tricks on how DMs have been using them.