Ditto on the attorney answer.
As for billing practices, I used to do 50-50. But some projects were only a day, so I'd just invoice the full amount when done. I never had a problem with payment, but the lag in receivables hurt. So last year I changed to:
20% at time of contract signing, to reserve place in schedule. I won't reserve schedule space without signed paperwork and money.
50% the day we begin.
30% invoiced after end of project, net 15 days.
This way, you already have all your costs and maybe more covered before your shovel even hits the dirt. I can't even explain how much of a burden this took off of me last year. I never had one anxious moment, all season long. Now, for a 1/2 day project, we'll take the 20%, and just bill the remaining 80%. By the time they bothered to get out the checkbook for the 50%, we'd be done anyway.