Step 1 is to protect yourself with a clear scope of work. Write up a clear, concise scope of the work you are performing. Include not only the work being performed by also what is NOT being performed. These are called exclusions.
Your scope should also include the price of the project, that billings will be done monthly on the XX day of the month and are due Net 30. Net 30 is a typical payment cycle that just about any company should be fine with. You should spell out how you will arrive at what you are billing monthly. For example, I use the percentage of completion method with billings on the 20th of the month. This means on the 20th of the month, I project ahead to the end of the month how far the project will be complete. If that is 50%, then I bill the client 50% of the project, less any previous billings.
This allows you to get your billing out earlier while still accounting for your expenses through the end of the month and therefore be paid earlier. You should always be ready to justify your bill. I strongly suggest a followup 10 days after sending your progress bill. It should be a very friendly, "I just wanted to make sure you received my progress bill and if you had any questions". This helps remove obstacles to getting paid on time.
You should also consider putting the time the project is to be completed and document any delays on the part of the company you are working for. Delays in work being performed that are not of your making cost you "opportunity costs"; the income for work you did not get to do for other clients or that you turned down because your time was reserved for their project that did not happen as per the schedule agreed to. However, this is a two way street so think it through.
Finally, your scope letter also needs to spell out how you will handle changes to the project (a change order), whether additive and deductive. The best way to protect yourself is to have language that says all change orders must be approved in writing before work commences.
Then whenever a change occurs you need to be proactive with presenting the change in scope (the change order) and the amount the change will increase/decrease the original dollars.
It is always hard to walk that razor's edge between losing work and protecting the work you are doing, but some opening friendly language on your scope letter about how the document exists to outline your mutually beneficial agreement, etc. can help the client see your professionalism.
Any professional company should understand the protection afforded for both parties by a clearly defined project through a scope letter.
You may want to get in the habit of having your bids also be the scope letter to save a step. In this manner the client just needs to sign off on your bid and you are ready to move forward.
Hope this helps.