How you bill is as important as how much you bill. The same piece of work can be great for cash or terrible for it, depending on the model you pick.
There are four common ones. Let's take each: how it works, and the risk.
Hourly. You bill for time spent. Low risk to you, because overruns are paid. But income is capped by hours, and clients dislike open-ended bills.
Fixed fee. One price for a defined scope. Great margin if you're efficient; painful if the scope creeps and you're doing free work. Protect it with a tight scope and change requests.
Retainer. A set fee each month, ideally billed in advance.
A £6,000 monthly retainer, billed upfront, is £6,000 of cash before you do the work.
Predictable revenue and healthy cash. The risk is scope: retainers quietly expand until you're over-servicing.
Performance. You're paid on results. Big upside, but you fund the work now and get paid later, if at all. The riskiest for cash.
Line them up on one question: when does the cash arrive?
Retainers billed in advance are kindest to cash. Fixed fees depend on your payment terms. Performance deals can starve you while you wait.
Match the model to the work and to your cash position. And whatever you pick, protect the scope; that's where the profit leaks.