You wouldn't think there was much to say about guitar straps. However......
Get yourself the best guitar strap you can afford and it should last you a lifetime.
The Finest Custom Guitar Strap ever - (I have mine made from here - approx £60)
One piece leather straps are generally better, but thin ones can not only dig into your neck over long periods of time - they can eventually tear and break.
The ends of a guitar strap are particularly important. Plastic ends sewn into fabric will almost certainly rip and tear over a relatively short period of time and should be avoided.
Two inch straps are standard, the wider the strap, the more the load will be spread and therefore the less they will dig in.
If you are on a budget, Levys make particularly good straps for the money