I think fantus that because the doctor jumps around in time so much, it's just that each time they come across each other he happens to be a little older and she happens to be a little younger - that is how we will see it in the doctor's timeline anyway. Like how he can go to the 51st century one week, and the 15th century the next. I don't think River jumps around so much because her method of time travel (the wrist strap thing) is less reliable than the TARDIS.
But it was explained in one episode that they don't always meet in exact reverse. So say she didn't move around at all, if she was born in 1973, the doctor might come across her in this order going by "his" timeline ie the order we see things happen on the programme:
2005
2003
2002
1997
2000
1996
1992
2004
1989
1988
So at his first meeting, she has all the memories from 1988-2004, but he has never met her before. Then it goes on with him having met her once or twice and she still has most of the memories of him, through to a time where they both know each other well (which it seems to be coming to now in the series) and then it will tail off again, he'll start to meet her younger and younger, if he saw her in 1992 (in my timeline) she'd only have met him twice before, and when he comes across her in 1988, she doesn't know who he is at all but he remembers everything about her.
Although I have a theory - and I hope - just from what River said in the library episode, that the last time they met up they had a picnic and it was perfect and he cried. I think that will be one of the very last meetings, something he remembers her saying and chooses to go back and do as a sort of last goodbye. - hence the random 2004 in there. That would be really nice as both of them would be in the same place for once, a bit sad though.