User higher guage strings and change them regularly.

Heavier pick?

But ulitmately, there are limits. When I play my openback banjo at my local session I simply can't be heard unless I'm hitting the strings as hard as possible, using metal fingerpicks, and even then only on the high notes. Means I only play right at the top of the neck on anything other than a quiet ballad.

So at that session I use a Danelectro Honeytone amplifier, with a cheap Maplin's clip-on cardioid microphone (it clips easily onto the banjo's rod). The amp's small enough to clip to my belt: if you couldn't see it you wouldn't even know I was amplified. All it does is makes sure I'm audible.