Since I didn't see a ton of comments posted by people who have experience with this, I thought I'd add one.
Delayed payment is normal. Large companies have very complex rules about how to make payments and how to process invoices. You must be extremely persistent and gracious in order to get things resolved. Each company will respond differently, but I encourage you to make use of some (if not all) of these following tactics:
1) Get a "Master Vendor Agreement" in place with the customer that states invoicing terms. This contract may take months to negotiate and require guidance from a lawyer. Once this is done, all of your projects should be addendums to this original master vendor agreement. This reduces the amount of paperwork the large multinational company has to do to validate each of your invoices and speeds them up.
2) Provide both a discount for early payment and a penalty for late payment. Annotate these discounts & payments on each invoice. If you carefully track your effort, you can know how much it costs you to track long term overdue payments. You can use this to determine how much of a discount you can offer for prompt payment.
3) Designate someone within your company as the "Accounts Receivable" person. It is their job to contact each customer with an overdue payment once per month (or week). They should very carefully take notes on all of their conversations and correspondence, but they _must_ be friendly and relaxed. The goal is to establish a positive rapport with the "accounts payable" person on the other side. It may take dozens of polite phonecalls to get routed to the correct person though, so you absolutely must be willing to put in the effort while not creating bad will.
4) Be gracious when payment is offered. Many times, companies may refuse to pay late payment fees; you can simply say, "I'll remove the late payment fee if you wire the money by tomorrow".
5) If desired, you can contact your bank to find out if they will finance your receivables. Some banks will provide you with cash up front (and charge you a fee) since they know how this process works.
6) Don't harass your point of contact until the invoice is more than a reasonable amount late. Generally, in a big company, the person who signs the contract doesn't even know the person who actually pays the bill. You want to avoid harassing your point of contact (who is usually your biggest fan) until you really need their help getting the bill paid.
7) Know your customers' "approval limits". Generally, executives at a large company will have specific approval levels - for maybe $500 they can simply file an expense report; for $2500 they have to file one form with one signature, and for more than that they have to get approval from a VP level person. If you can keep your projects small enough, you can bypass some of the challenges.
8) Once you've read lots of advice on slashdot and picked a strategy, contact a lawyer before doing anything. Most lawyers will be able to confirm whether your plans follow the law quickly. It'll only cost you a small amount.
And finally, remember, "managing receivables" is part of the cost of doing business with large companies. Factor it into your project costs.