As many will tell you, one of the big things about keeping a white car looking its best is to make sure all the trim is kept in tip-top shape. The contrast between the white paint and the blackness of the trim, tires, wheel wells, etc. is what really make the car stand out.
That being said, realize that it's very difficult to get a white car to "pop" like it is for darker colors. It's really very subtle, and while white great for hiding minor swirls and scratches, that also means its harder for you to correct any defects that might be there as you won't see them as clearly.
My advice to you is to first polish the car to remove any marring that might be there. Follow that up with a good sealant of your choice, and then finish off with Natty's Blue (which I've found to be very nice on my white car). Make sure all the trim and tires are as dark as can be, and don't forget the glass either, as this can make a big difference in the look of any car, not just white.
This was my original starting off point, and while you can then branch off from there, this will give you a good base to work from.