A few things I wish I'd known sooner.

Via Lifehacker comes a pointer to Tricks of the Trade, a blog containing “Professional secrets from those in the know.”  Some of my favorites that are applicable to other professions (well, at least some of them are):

Police Officer:   Here's an easy way to know if people are lying: they touch their faces. Even if they know not to, most people when under pressure and telling a lie will tug on their ears or scratch their noses.

Residential Developer:   Here are some ways to make even the smallest condo seem spacious. Remove all interior doors -- no one notices, and hallways and rooms immediately feel bigger and more open. Use custom-built furniture scaled down for the space. Glass and lucite tables 'disappear,' and large framed mirrors magnify space. Lastly, print take-away floor plans on huge sheets of quality paper -- even a 450 square-foot studio suite looks exciting and big on an attractive 11x17 sheet.

Professor:   If you have to give a poor grade to a student you know is going to object, put a lesser grade beside it on the paper and then scribble it out (but not so much that the student can't make it out). It will make them think that you originally gave them the lesser grade but then raised it after some thought. 99.9% percent of the time this will prevent them harassing you.

Bellboy:  When you let guests into their rooms, ask to use their phone, pretend to dial the front desk, and say, "I'm in room 204 with Mr. and Mrs. X, and I just want to make sure that the problems we've had with the loud guests in room 304 have been resolved. Because otherwise I'm going to move these people to 233. They checked out today? Excellent, thank you." Hang up, smile modestly, and get ready for a large tip.

Adult Music Teacher:   Life is too short to spend with uninterested students who rarely practice and never make progress. To "fire" these students, just say, "I know you're very busy and don't have much time for practicing right now. Tell you what: why don't you call me when you feel prepared and we'll schedule a lesson." Nine times out of ten you'll never hear from them again.

Painter:  Before resealing a can of paint, blow one deep breath into the can and close it quickly. You'll fill it with carbon dioxide, which will keep the paint from oxidizing prevent it from developing the "skin" that paint gets when it sits a while.