1) Know the clientele you are targeting. If you try to pitch Celtic harp to a venue that has regular mosh pits, you're going to be laughed out the door. Similarly, if you sell a death metal group to a light listening lounge or hotel reception area, you're going to be shut down.
2) Have a marketable product. Have demos, preferably with video available. You may be descended from Apollo himself, but your potential bookings will not know that. Be humble, and let the music speak.
3) Remember what your job is. You love your music, but the venue couldn't care less. What DO they care about? That your service helps them sell their service. Whether you're playing harp in a lobby to help a hotel sell rooms or playing Mustang Sally for the 1000th time in a seedy dive bar to sell Budweiser, your entire point of being hired is to help your present employer sell their product.
4) Build a fan base. This involves a lot of hands-on marketing. This involves talking to people at shows. This involves knowing how to communicate across barriers. More fans means more shows and/or the liberty to be SELECTIVE in your shows! No hanging out backstage with the drunk busty babe doing lines of coke of a clown's boner.
All in all, use common sense. I would further recommend that if you want to make a side gig out of this, you should legitimize it as a business. Get insurance and a completely separate bank account. All money goes through that bank account. File everything on taxes, save receipts, etc. Don't forget to 1099 your band mates as contractors.
Source: Life experience as a 29 year old cover rock musician actively playing every week in every shithole in North East Florida.
