Recognize the runaround and put a stop to it as soon as possible.

The human condition is one of flux. One of the nice things about lending to a friend is that, hopefully, you can be a little more flexible than a big, faceless corporation. However, if your friend is taking advantage of your good nature, you will not be helping him or your friendship by letting your friend slide on their repayment.
John borrows $150 from you and promises to pay it back in a week. At the end of the week, he lets you know he will need another week because had other expenses. You agree to let him have another week. A week later, he tells you his pay was reduced and he can't pay yet. Meanwhile, John has plenty of money for food, dinner and entertainment. He isn't taking the debt seriously, and is just running you around, and the sooner you recognize it, the sooner you can deal with it.
If John is just a kind of strange, you may be best off to consider that money gone and just resume your friendship, deciding never to "loan" John any money in the future.
If you really must have that money back, you may need to face the fact that you and John are going to have a serious fight, or maybe aren't going to be friends any more once you have your money back. Think carefully about how you want to handle this situation.