For acrylics Dettol works fine, just leave your figures to soak for at least 24 hours - if it doesn't work totally the first time round just give them another go. Oh, and don't throw the used Dettol away as it's reusable.
For enamels I use Acetone which works really well, Superdrug sells their own brand quite cheaply but make sure you get the real stuff as they do sell a weaker 'ersatz' brew.
For Heyer, this is the stuff they sell in the U.K.
(http://img600.imageshack.us/img600/3820/6gzq.jpg)
If you get any paint left in nooks and crannys, which tends to ball up when you try to remove it with needle or such, sprinkle some talcum powder on your dry mini and it'll come off a lot easier.