It is always a really hard thing when chasing money from clients. You want to keep them as clients and really, you want to be sure you stay on their very best side so they think of you again in the future; however you cannot work for free. Bills must be paid. You provided the service and it is only right that you be paid accordingly for it. However it doesn't mean that makes it any easier when you have provided the goods, sent an invoice, made it payable after 30 days (which is a long time for a freelancer!) and then it is another few weeks before you see the money, and that is if you squeak away like a rusty cog.
It is unfortunate as well that the larger the company, the more they seem to think they can get away without paying you on time. I in all honesty find it really hard work. The only thing I can possibly suggest is to be polite with your chase emails. Email once then phone. Try to be put through to the finance department first rather than your contract as it may be the case that they are not even aware the invoice remains outstanding. If you have no luck with the finance department, the emails and the phone calls then try your contact.
Also, think about this carefully, really carefully but as a freelancer you can charge interest. You are allowed to charge 8% per annum on any outstanding amounts. I wouldn't recommend putting that on your first invoice to a client but if needs must; you can add it to a third or fourth invoice if it seems necessary. Just think about who you are invoicing and go about it carefully.
If any of you guys have any tips that you can recommend then please.....I am open to ideas! Show me the resources to show me the money!!