This is an easy problem to solve! Because you are currently performing work for your client, your work is currently valued very highly by your client, so you'll use this as your leverage by following these tips:
REMAIN CALM, STAY UPBEAT AND POSITIVE ABOUT GETTING PAID
Remember, you are going to be paid no matter what. This is what you must believe when corresponding with your client by email or by phone. You don't want to let on that you feel threatened that you're going to be out hundreds or thousands of dollars if they don't pay you. Keep your words possitive, and only mention it once in the email, and say you are looking forward to continuing you work.
STOP YOUR WORK
Fortunately, if you are not getting paid by a client who pays a monthly retainer, you have the upper hand. You can stop your work. Your client may be cracking the whip, and sound upset if an action is not accomplished that they delegated to you. They may even tell you how offended they are that you only care or work when they pay you (true story...I've heard this many times from business owners).
But in the end, the client is not abiding by their end of the agreement which is to pay for services rendered. If you are not in a monthly retainer position, but are in a project that involves partial payments to be paid as checkpoints are hit during the project, such as a website design project that has an innitial deposit, a payment point at signoff of the design, and a payment point upon launcing the website, then you must stick to those checkpoints and bill your client. Give them the stated number of days to pay (this should have been stated in your original agreement or contract that they signed). If they don't pay, kindly tell them that the project will pause until payment is received.
ACCEPT CREDIT CARDS - CHECKS IN THE MAIL DON'T COUNT
If your client let you know that a check is in the mail, that's all well and good, but it's not in your bank account, and you can't pay the bills you need to pay. If you've stopped your work, only resume the work when the check clears in your bank, and not before. If you resume work when you get the check, and you sit on the check for a few days, and then deposit it, you'll be really upset when that check bounces and you are not only hit with a bounced check fee, but you've done more free work for your client. And yes, this is an example that I have seen numerous times before happen to fellow professionals.
Use PayPal, Chase QuickPay, QuickBooks or Freshbooks payment features, or any other invoicing tool you have discovered. You may need to upgrade your PayPal or QuickBooks in order to send invoices, but doing so is 100% worth it. Most likely, you will also be able to key in credit cards into your computer when your client gives you their credit card information over the phone. And they will give you their credit card information because they will want you to continue working.
EMAIL WITH KINDNESS
Because you are accepting credit cards, here is a model of what you can say to kindly let your client know that they need to pay. Send this type of email after they have not responded to you sending an invoice for about two days, or if they have agreed to pay but still haven't.
Yes, I can continue working on that with you once we get caught up on your invoice! I've included a payment link for your convenience, which is a link that you can click to pay with a credit card.
Looking forward to resuming this project with you!
CHANGE YOUR BILLING METHOD TO BEING PAID IN ADVANCE
If you aren't already billing your client in advance for retainer work, you need to change that right now. The easiest way for your client to pay you in advance is to set up auto-billing that recurs each month. See my article on how to do that with PayPal's recurring billing tool. If your client owes you money, and you want to prevent it from happening again because chasing money is a waste of your time, here is how you can phrase this in an email:
Yes, your invoice does need to be paid at the beginning of our retainer cycle, so as soon as you pay that using the payment link below, we can get started on this new item and can continue the other projects we are working on.
I should let you know that starting this month, our billing structure is changing. To keep our costs down, are offering a new payment method that auto-bills you each month. To take advantage of this, we are offering a 10% discount on the monthly retainer for those who move to this subscription method. If you would like to start that, I've included the payment link below. You may stop it at any time, but this way, it's easier for you and you won't miss a payment!
If you are a solo-preneur, you have only yourself to pay and collect money for. This can make you feel like "Oh, I don't really need the money...I just want it, but I guess I can let this slide for a bit." WRONG. Businesses who have employees or contracted workers owe their employees payments each month. If they have payroll to meet, that payroll is being withdrawn from their bank account each month, so the money needs to be there. The cash flow needs to be up.
If you get into the slump that most freelancers get into, where they don't feel "worth it" when emailing their clients about collecting cash, or fear offending their client, step away from this line of thinking right away. You are entitled to get paid, deserve to get paid, and if you had staff to pay, or bills due, these self-defeating, cobwebby thoughts would quickly blow away. If you don't communicate clearly to your client, you will end up resenting them, and that usually produces poor communication which won't get you your money. So keep it upbeat, keep it positive, know you will get paid, and proceed with confidence. If you need to let them go as a client, then do that. If they aren't paying you, then they aren't a client anyway. Part of the definition of a client is someone who pays for something. So...until they pay up, they aren't a client anymore.