There are a couple of different things that can help with cash flow:

1. When working with a client, request a down payment up front and/or do incremental billings so that you have cash flow throughout the project vs. just at the end.

2. Supplement your long term projects with short term to mix the cash flow cycles.

3. Depending on your credit and length in business, some businesses find it helpful to get a line of credit to help during the slow months. Just don't rely on this all the time or the cost of financing may eat up your profits.