...Is there any way to tighten up the connections to make a better friction fit?
The trick to that is to have the terminal that slips into the connector as straight as possible when you slide it in, and to leave sufficient slack in the wires so the connection isn't under any tension. Some crimpers will bend that terminal at the point that the crimp part of it ends. If the terminal is straightened, the leaf spring in the housing that "locks" the terminal in will provide sufficient upward pressure to keep the connectors together--unless the wires are too short or simply under tension.
In that case, a thin zip tie can be used to keep the connectors together, just thread the zip tie between the wires of each side of the connection and pull it tight. That defeats the "quick disconnect" feature of the powerpole connectors, but it does work.
The ideal is to not have the connector under tension but, circumstances do not always allow this. I find that these make a more workable solution than "tywraps".http://www.powerwerx.com/powerpole-accessories/powerpole-retention-clips.html
Powerwerx Retention Clips for PP15/30/45 Powerpole Connectors
Secure Powerpole connections in the field, shack or mobile installations. Easy to use and ensures a positive connection if the chance of Powerpole separation exists. The wedge shape provides maximum grip for removal. Direct replacement for Anderson 110G68.