For an invoice, I just use a generic NEBS form.
For service calls, I don't get a contract signed. Just issue a receipt/invoice when done, unless the "service call" really turns out to be a construction project. You get surprised sometimes when you show up at what the folks had in mind.
For a residential proposal/contract, I use another generic NEBS form.
For a comercial contract, I use a slightly modified version of
this.