(Personally tested and recommended by the author of this web. Some links might lead to other websites.)
In the past, every now and then I was too warm-hearted and let people persuade me to take jobs I hated. From full-time jobs when I was looking for employment, to small contracted job when I was self-employed. It always had a fatal effect on my life. I couldn't do these jobs, I couldn't do other things, my life seemed to stop, everything got freezed and nothing done. I knew that would happen, but I still said YES. I was saying yes way too often. But I have learned. So should you.
Saying NO in the right time, can save you hours, weeks or even years of lost time. It can help you move away from things you hate to things you love, from procrastination to productivity, towards better and more enjoyable life. Don't be afraid of saying NO, quite the opposite, learn to enjoy it and feel proud and strong everytime you say NO.