One challenge I've had is riveting flanges against a webbing or similar cases where the outer surface wants to lift up to create a gap between the two pieces of material. I made a few observations here and have found an approach that seems to work pretty well in a variety of circumstances.
When able to back-rivet, I've found this to be the easiest as the back-riveting tool has a strong spring that compresses the pieces together before riveting.
When using the rivet gun and bucking bar, unless you press the sheets together with your bucking bar hand a gap will often form between the sheets. This has caused me a lot of early frustration, as once this happens the sheets become slightly deformed and even after drilling out the rivet it becomes even harder to get them compressed flush without a gap.
The technique that I've found easiest is to get a piece of rubber that has a hole punched in it just large enough to hang on the end of an unset rivet. You have to be careful on the thickness of the rubber; if it's too thick the bucking bar may not make contact with the rivet shop end. If the rivet is just slightly set with the rubber to compress the pieces, they will hold together. Then the rubber can be removed and the rivet fully set normally.
When rivets can be squeezed, I've found having the rubber insert works well here as well, but doesn't need to be removed to fully set the rivet. I've also found that the rubber pad tends to square up the squeezer to be orthogonal with the surface.