I'll second Freshbooks. Awesome for invoicing, receiving invoices and now importing expenses from your cards & accounts. It pretty much handles everything you need for collections, receivables and taxes.
I'll second LinkedIn as well. Keeping a network of people you've worked with and paying for the business account allows you to find (your network * 3) of potential sources to introduce yourself to. Ihaven't tried the jobs feature - will do now that you've recommended it.
Recruiters work too. They are expensive to your bottom line but mitigate risk. They typically have 6 month waiting periods after you've finished a contract before you can work with the client directly - and I recommend you wait patiently then let the client know you are now independent.
I'll finish with this: know who you are targeting. LinkedIn has an industry and location search - your acquisition strategy should too.