Saw this on an FB post and thought I'd answer them here.
1) How do you handle a TPK?
2) How do you get players to do their homework?
In my 30-something years of experience as a DM, I've learned that players do not do homework. Especially American players. And that goes triple for ones under 40. This is understandable however as 'let me tell you about my campaign setting' is the DM equivalent of 'let me tell you about my 12th level Paladin' and I don't think a lot of DMs have realized this(I know I've been guilty of this). Sometimes it's unavoidable however, at which point my advice is, keep it brief. No more than a 3 question thing. Offer benefits for completion.
3) What are some good monster combos?
The MM and Role Playing common sense handle this pretty well. Beyond that, I don't worry about it. Creating 'killer combos' always rubs me the wrong way as a DM. I can literally Herd-of-Tarrasque the PCs if I want to. I'd rather create interesting locales. Players who actually explore will be rewarded with stories for them to uncover that can lead to riches and power. They just have to put their detective hats on.
4-6) How do you DM line of sight vs. line of effect? How do you handle knowledge roles and player metagame knowledge? When do you allow skill substitution during skill checks?
The rulebooks are usually pretty clear, and when they are not, common sense usually prevails. In the incredibly rare instance in which this doesn't happen, I roll a die or have players roll Luck.
7) How do you handle parties that are a combination of newbies and more experienced players?
Like any other game. I expect the old hands to show the n00blets the ropes. It's in their own best interest that the rookies don't do stupid things. Plus, I have never met a veteran player who didn't like to flex his/her knowledge and skills.