I create an invoice immediately.
Start invoicing after the first conversation where you agree you will be doing the work.
Invoice immediately.
Micro invoice if you have to.
Invoice in minimum four hour blocks just for talking.
Invoice in ten+ hour blocks like a retainer for mom and pop shops so you can inch forward.
Invoice requesting portions up front (just to begin!) on invoices worth over 10k.
Bill reguglarly.
Invoice up front for the TOTAL amount, and then send the same (modified) invoice periodically (once a week, once every two weeks, every 30 days, or on incremental delivery. What ever works best!) for regular payments on completion of milestones.
And did I mention KEEP INVOICING! Don’t have feelings. The contract part is you will DO THE WORK and they PAY YOU.
You may think I’m being silly. Too many fine technical folks are ignorant and afraid of MONEY.
Bill for everything productive (talking, email, meetings). In fact, a good rule of thumb is to add 25% to your estimated project costs for TALKING and other administrative tasks (as low as 15 minute increments if necessary.)
Good luck! And remember, INVOICE!
Consultancy, mentioned here, might be a useful stepping stone to get the feel of being a supplier.