Seek out new economic buyers laterally during your projects. It is better to do something pro bono than to do it for a low fee. Fees have nothing to do with supply and demand, only with value. If you are unaware of current market fee ranges, you're undercharging. Psychologically, higher fees create higher value in the buyer's mind.
Value can include subjective as well as objective measures.
Introduce new value to existing clients to raise fees in these accounts. When forced into phases, offer partial rebates to guarantee future business. Start with payment terms maximally beneficial to you every time. Offer incentives for one-time, full payments. Never accept payment subject to conditions to be met upon completion.