Welcome to contract work!
Customers paying late is a fact of life, and from time to time you'll need to be persistent and/or agressive about collections.
Get in touch with your contact at the client company and let them know you haven't been paid for your work yet. Ask for a contact in their Accounts Payable department. Follow up with that person and make sure that your invoice got entered into their system. Make it clear to them that they're past due, and unless you receive payment on the next banking day that you will begin assessing the late fee you stipulated in your contract (you did stipulate a late fee, right?)
If you're dealing with quality clients, they'll almost never just try to stiff you on the payment; instead, it's because their accounting processes are completely disorganized (sometimes intentionally!). If your client isn't a large, trustworthy, stable company, always get your payment up-front. Even if they are a big company you trust, get an advance; it'll help tide you over while you're waiting for them to pay your net-15 invoice... net-90.
After consulting for more than four years, my past-due receivables balance (which at one point exceeded $200,000!) finally went to down to $0 only last week. And that's because I stopped taking on new work.
I've had customers who were nearly 90 days behind on payment (I used to be really bad / lazy about collections) and required almost a dozen followup emails and calls to get things straightened out. As far as I can tell, it was never out of outright malice, just the general disorganization and reluctance to part with cash that you find at big companies.
Don't take any aggressive / adversarial actions until you've exhausted all the polite request options. Give them the benefit of a doubt (unless your contact is already the person who writes the checks, and has no excuse). Be polite, firm, and extremely persistent. Get a commitment that you will be paid before date X, and if you haven't been paid in full by then, assess your late fee and start calling / emailing on alternate days until you do get paid, or until your patience is exhausted. Get lawyers involved only as a last resort. And next time, get paid in advance. :)
