An excellent book I read about assertiveness
www.amazon.co.uk/When-Say-Feel-Guilty-Systematic/dp/0553263900?tag=mumsnetforum-21
Best fiver you'll ever spend, and I use these skills on a regular basis. Doesn't mean I'm not shaky, or feel anxious when dealing with confrontation, but having the skills, and a script (I guess?) definitely helps. Also, the first time you use the skills, and the conversation goes the way you want, it gives you a huge boost instead of quietly seething as you've said yes to something you wish you hadn't
Some other nuggets of wisdom I've picked up:
No is a complete sentence.
If someone is trying to get you to do something and you don't want to, don't give an excuse. That's just starting a dialogue - eg
'Can you babysit my kid?'
'No, I have to take my own kid to the dr'
'Oh but you could take my kid with you'
'No, it would be easier with just my own kid'
'Oh i know, but if you could just take her/him it wouldn't really be that tricky would it?'
etc etc until the more persistent one (hint, not the non assertive one) wins.
In contrast:
'Could you babysit my kid'
'No, I'm sorry (if you like, to soften the no) I won't be able to'
'Oh' (silence - don't feel obliged to fill it. There will be mounting social pressure, don't give in, smile, breathe, wait it out. Don't cave. Don't. Nope, no caving. Just wait.....
'Oh, ok, I guess I'll have to find someone else'
(This might take a while, and people will argue, but keep your cool, or look like you're cool anyway, and just keep repeating yourself). If you feel like you're being rude, are you the person haranguing another to babysit your child? No, you're not, they are, so who's the rude one?
Try to care less about what people think of you. That's a harder one, but if you're aware of your flaws (to quote a certain GoT character) no one can use them against you. You know you're not assertive, so you're working on yourself. Any cheeky fuckers who've taken advantage of this don't want you to change, and they will use tactics to try and keep you doing their bidding. Ignore them. True friends will want you to be the best version of yourself, and will support personal growth I'm sorry, that's a terrible phrase, I'm ashamed of myself for using it 
