If you can, please DI your signal rather than mic'ing. The sound quality really will be better.
If you have to mic,
ALWAYS wear headphones for monitoring, and
make sure the level in your 'phones isn't so high your mic's picking it up (it's better for you and better for those you're jamming with).
Before joining in, play along with "local transmit" turned off, just to get the feel. Don't forget to turn it on again, though. (A perpetual source of embarrassment...
)
If you're not happy with the mix you're hearing, check the previous post first, then alert the people you're jamming with if you're sure they need to adjust their levels. As a last resort, use your sliders.