1. [ Small jobs ] All work should be billed for and the client should be made aware of it. Make sure that you have a paper/email trail to fall back on.
If the customer calls with "just one quick change", politely ask them to flick the request to you in an email and you'll add it to your job queue. You don't want customers to feel they can call you up and have things done right away or you'll soon be doing nothing but talking on the phone.
If it's only a small amount, keep a running total and send them the bill at the end of the month. When the small job is done let them know that you added 30mins or whatever onto their monthly invoice.
This way everyone is kept happy. You can do quite well out of the small jobs if they're managed correctly.
2. [ Free Advice ] Same as above really, you need to get them off the phone and politely ask them to send an email with their questions, then you will add them to the work queue to be completed and billed.
I've found that most people who try and get free support/advice and just trying their luck and are more than happy to pay once it's made clear that you cannot offer free support. If they don't want to pay, then you don't give the support.