When I have a new harp student, I use a contract which I basically copied from my own teacher, and it's worked very well. To me it sets up some ground rules about safety etc (essential in my case when I am teaching children) but to be honest I've never actually had to use it for anything other than sharing some expectations at the start of a process about how both children and teacher are protected.

I've never used one when doing video work (and I even feel bad saying that), but I guess it could be helpful - for example, if you draw one up with a client, it could generate a useful conversation about what is deliverable and when. Hopefully, though, most people would have those conversations anyway, and discuss any subsequent things in an email - because of course, client expectations change.

A simple example: if you say to someone "what is this video for, where is it going to appear?" they often don't know, but during the process, ideas can emerge. As long as you both understand that there is a cost attached to everything, and feel happy to discuss that, then I don't necessarily see that a contract is vital for that. However, it might be very useful elsewhere: for example, to state who owns the finished product, who has copyright etc.

I remember someone once saying "A failure is usually the result of bad contracting" - but that doesn't have to be a written contract. I think a failure to get good results is more usually the result of a bad relationship, either because you haven't established the right things at the start, and/or you haven't kept up the communication. It's like sailing: the direction you set at the start can have enormous implications later, but unpredictable things can happen, and even if you have set the right course, you have to keep your eye on the destination and make corrections as you go.

So many things to consider:

a) Nature of business arrangement
b) Legal jurisdiction - country, province, city
c) $$ value of the deal & intellectual property rights
d) Legal entities - are you and they individual entrepreneurs or LLC entities?
e) Enforceability - if you are David vs Goliath and don't have the money to fight it out in the court system, maybe you want to use agreements or Memorandum of Understanding (MOU) instead of contracts. Contracts cut both ways, if in theory you can sue the pants off your adversary, they can stick it to you too and more effectively if they have big bucks. MOUs are not legally binding but can be used to clearly list both parties' responsibilities and obligations, performance and deliverables, ownership of work in progress, etc, and do not prejudice your right to take them to court later if you want to.
f) If you live in place with a court system biased towards big business and people with VIP political connections, you're effectively screwed.

Rather than try to create a watertight contract, arrange the terms of the deal in such a way that they need you and you can't lose.

Since you didnt say what you're doing , my guess is that if you're doing some kind of service over a period of time then get progressive payments for every milestone reached. If there's no payment then there's no further development and they have nothing.

Price your rates higher so that by the time they pay you 85% of the contract value, you've actually gotten 100% of what you originally wanted. So you don't lose out if the client then decides to stiff you over the remaining amount (or requires that you warranty your work for a year before you collect the last 15%).

And always include a variation clause - if the client changes the requirements after the terms have been signed and sealed then there should be allowances for extra time, increased project cost, and right to refuse changes (or at least right to renegotiate terms).

Force majeure, acts of God, war, strikes, riots, and civil commotion clauses can help or bite. Use with discretion.