I don't want to sound simplistic, but have you tried squeezing (bending) the prongs together so they are less parallel and closer at the end that enters the wall? That is really the only solution I've ever tried because it has worked well enough for me.
You could try to either apply light pressure to push the prongs together or apart.
What I usually do is just squeeze the prongs inward just a little; this allows for a firmer grip.
Best advice-REPLACE THEM. Receptacles wear out like anything else. Working for an electrical distributor I can tell you there are different grades of recepts. If it is one that you are going to be plugging into and unplugging a lot, like where you plug in portable devices, get a better grade. Squeezing the prongs to get better contact is not a good idea as you are lessening the contact area with the recept which creates more heat which in turn shortens the life of the recept and in very bad cases can cause a fire. In these times we all try to save money with shotcuts but we should spend what is necessary when it comes to doing things safely. It you aren't comfortable doping it yourself, get an electrician, it is money well spent.