Well, you can prove that they were there by simply having them on the car. So that part isn't hard. Proving what they cost may be another matter, and it's not easy without receipts. So keep them somewhere if at all possible.
If you get really overboard, like my car is, and the value of your mods becomes a significant portion of the value of the car, your insurance company may request that you get your car independently appraised so that there is written proof and a professional opinion of the value. This is particularly important if you get high-quality work done that might be unusually high-priced by your insurer's standards where they may question the validity of your receipt -- i.e. a very expensive paint-job.