Remember: If you make yourself a doormat, people will walk on you!
For those people who instinctively listen and say yes, here is a strategy I used to use when I was a supply teacher and a school would phone to ask me to work on a particular date. I used to say yes quite automatically if there was nothing on my calendar for that day but some schools were very tough to work in and I wasn't that desperate! I simply developed a habit of saying, hang on, let me fetch my diary. This gave me breathing space to formulate acceptance or not. I wasn't lying when I then said, I'm sorry, I'm busy that day (nothing more than that was needed).
So, for the friends you think might want to 'take a lend' as we say in the north east, you can have a few instant, delaying tactics at the ready before they even embark on their sob story. Have a sticker on your phone or a note beside it that simply says hang on a minute! Say that and walk away from the phone and count to 30 to give yourself time to strengthen your resolve (you can invent a reason if you like, kettle, cake in the oven, pot boiling over, someone at the door) then you can come back ready to listen. You can then use the calendar/diary delaying tactic if there's a date they need you. You can be busy that day, the same afternoon etc. if necessary. If you don't mind a white lie, you can have a fictitious appointment but saying no without explanation should be enough. It would be a very rude friend who asked what you'd be doing so they could decide which was more important, your dental appointment or their problem! Also, it puts the other person on the back foot if you disappear for 30 seconds before they get going and again to fetch your diary. I don't keep a diary but I could start at any time ?
A phone with caller ID is also a help so you can see if it's family, real, trusted friend or potential user! You can then decide whether or not to answer. If the problem is urgent they'll leave a message. Sounds harsh? I don't think so. There's nothing wrong with being assertive. A bit of practice and you'll get better at saying no.