are you stretching the strings? it really helps enormously to do this, you just firmly but not violently pull the strings away from the body of the uke, holding them around the soundhole sort of area. when you first get a uke, or change the strings on it, get in tune, then stretch each string a few times, then re-tune, then repeat a few times, you'll hopefully see that the first time you stretch the strings they go out of tune a LONG way, but with each repeat they go out of tune less. i do the string stretching every time i tune the uke on the first day, and then at least once a day for about a week after that, after which time everything has definitely settled down nicely.
what size uke do you have? i recently got a concert uke, and after having no problems with aquila strings going out of tune much once they settled in (and they settled in quickly) on my other ukes, on the concert i continued to have to tune often. i changed to living water strings, and i have no problem now, so if aquilas don't work for you, try any decent make of flourocarbon strings