I also learned that the net payment terms on my contracts with clients were as meaningful as toilet paper. Getting paid in 30 days was surprising, 60-90 was more the norm.
Ultimately I just started changing the terms with new clients - telling them that we could work on a typical net 30 arrangement at a top tier rate or I would give them a discount to buy hours ahead of time in 100 hour blocks and I would burn down against this. Most heard 'discount' and nothing else mattered. It is difficult to change a client to this model but if you can start a relationship from this perspective things get much easier.