Tips:
1. Set your feudal taxation level to 'None' unless you're really hurting for money. Money is not hard to come by in this game, and the amount you'd get from taxing vassals is better spent in the relationship bonus. Your own demesne will generate plenty of money as long as you invest in money-generating villages and make sure you upgrade your own holdings (i.e. the capital holding in a county, not the cities/baronies/churches built there) first.
2. Yes, go with Medium authority. The best part of this is being able to revoke the titles of heathens/heretics for free. Revoking somebody's title and replacing them with a hand-picked vassal who has the 'content' trait and a high relationship with you will give you a staunch ally and remove a thorn from your side at the same time.
3. Managing vassals: give gifts freely. Again, this ties in to the 'invest in money-making infrastructure' thing, but I find that liberally lining the pockets of my vassals every so often will stave off the vast majority of problems way before they can occur.
4. Same-dynasty thing is okay as long as you make sure not to give titles to people who have claims against you, since that causes problems.
5. Make any changes to Crown Authority and Inheritance when your current character is well into middle or old age. Do NOT change laws with a young character who just inherited! BIG MISTAKE. They will immediately start out with a 'short reign' penalty, and if you add negative penalties for changing laws on top of that, you're going to have people rebelling left and right. Remember, change laws with old characters. Not only are they far more likely to be able to manage the discontent caused by legal changes (long reign positive modifier), but that penalty will also disappear entirely as soon as they kick the bucket and you start out as your heir.
6. GIVE GIFTS TO THE HEIRS OF YOUR VASSALS! I can't stress this one enough, and it is a hugely overlooked trick. It can be expensive to give gifts to powerful vassals. Usually this isn't a problem for me, because I invest heavily in money-making early in the game, but in the middle of a big war when you need mercenaries, or if you've just spent a ton of money on building, sometimes it isn't plausible to pay two hundred gold to some schmuck just because he has a few titles. But there's a trick you can use for managing your kingdom in the long term. Untitled heirs only cost *20* gold to bribe, and the positive modifier you get from gifting them will be just as large, if not LARGER, than the one you would get by gifting your vassal.
7. If your vassals are pissy because you have direct vassalage over a count who is part of their de jure dukedom (or a duke who is part of their de jure kingdom), transfer the vassal to them. It will remove the negative penalty and put a positive one in its place.
8. Likewise, try NOT to have counties that you don't have ducal titles in. Dukes get pissed off if you have control over a county in their de jure territory. Personally I prefer to target the richest duchies and just take the whole things for my personal demesne; that way I don't have to deal with either the 'wants control of...' or the 'desires the duchy of' penalties from anybody.
9. Prune the heirs of your vassals wisely. Let's say you have a powerful vassal with two sons; the heir (oldest) is ambitious and envious, and the younger is content. Which one of those two do you want to inherit? Remember: it's much easier to successfully plot against a young, powerless child or teenager than a powerful duke. Kill off that little snot before he grows up and causes you serious problems.
10. As a counterpoint to the above, if you have an old vassal who doesn't like you with only one heir who also doesn't like you, sometimes it isn't worth the trouble of bribing the heir if they have a lot of bad traits (envious, ambitious). Trying to bribe envious/ambitious people into submission is often not worth it. Just kill the heir and wait for the title to go to YOU when the bastard duke/count dies, then reassign it to somebody more docile. Problem solved.
Edit:
11. Oh, and one last thing. Sometimes, if you imprison a rebellious vassal, you won't be able to revoke all of their titles (for some reason the game will usually only allow you to revoke one - unless they're a heathen, then you can go hog wild). In that case just let them cool their heels in the prison for the rest of their worthless life, and either kill off their heir(s) or make sure they're well-bribed and happy with you.