Keep it brief
Cover letters should always be short and to the point. Be professional.
1 - Thank you for your interest in .... etc. (greeting, reminder of previous contact(s).
2 - We are enclosing .... (the body of the letter in a single paragraph, ie. what, why, when, etc)
3 - Please contact us at .... (call to action, specific next steps to be taken or question for them to encourage them to get back to you, continue or finalize the transaction. Again, a single short paragraph)
4 - Thank you (again) and closing. (keep it short)
Be sure all your contact info, phone, email, website, etc is on the header, footer or letterhead. Don't get long-winded or duplicate info already in your promo pack.
Keep track of mailing dates, calls, etc. If you don't hear back in a few days or a week, call or visit in-person if possible for personal follow-up while it's still fresh in their minds. We started a spreadsheet in MS Excel with columns for venue, manager, address, phone, email, dates of all contacts & mail-outs sent, and dates/prices from any previous gigs there. It's great to be informed and sound like you remember everything clearly from previous dealings.