Don't let any boys in there!!!! Seriously - that's why bathrooms smell.
If you have to have a unisex, then make sure the boys: a) wipe up any dribbles b) flush after every time c) don't have carpet in there - or at least cover it with a pedestal thingy... the mat which goes round the loo.
Failing that, loo blocks help a little bit... Not much tho - I still recommend bathroom segregation!
Use antibacterial toilet wipes to wipe down the outside of the loo, walls, skirting board, floor - anything which might be harbouring smelly bacteria. It works but obviously you have to repeat when necessary.