Days Sales Outstanding is one of the most critical financial metrics to keep an eye on when trying to maximize cash flow into your business.
Although we often say profit is queen, cash flow is king.
So you're right to watch your DSO.
At the end of the day, you really don't have any recourse beyond quitting.
Do you offer a service/product that provides you with a sustainable competitive advantage...something that your competition cannot offer? If not, you are disposable, and your customer (your employer) will continue to exact pressure on your accounts receivable. Likely, if your SCA is weak or non-existent, and the supply of goods/services you provide outstrips demand, then your customer knows they can go down the block and do the same thing to the next vendor (i.e. employee).
You have to find a way to become indispensable, or at least, more important, to your customer's bottom line. I'm sure your customer pays some vendors on time, because the fear of threatening that relationship with poor metrics against their receivables would threaten your customer's bottom line. You need to find a way, or you'll continue to suffer the consequences.
Perhaps look into the economics of the goods/services you provide. Is it possible your goods/services are going through obsolescence? Or perhaps there are so few barriers to entry for your competitors (read: other new potential employees) that the market is flooded with other good/service providers who've been able to knock off any SCA you thought you might have had. Perhaps you over-value the goods/services you provide...clearly, your customers believes so, otherwise, he/she would pay you on time.
Divorce your personal feelings from the situation, and study the economic motivations of your customer. You'll see the truth. Signalling theory, my friend. Don't listen to what customers say...watch what they do. Our signals betray our words and reveal our truest intentions.
You need to work on your SCA. Or change your business so that you can create one. Otherwise, you're too easy to copy and can be rendered obsolete before you think your brand lifecycle really should be done.