Do you know how to select cash games where you'll do best? Here are my tips from my nearly 25 years of experience:
1) The absence of good players is more important than the presence of bad ones. The very best game you'll find is where you're substantially better than everyone in the game, even if there are no outright fish. If you feel like you have "control" of the action, and everyone seems a good deal less skilled than you, then you've found a great game where you will win a lot of money, provided you run at least okay. Do not obsess over finding games with megafish or maniacs. You're much better off in a game where there are no players at the table who can compete with you.
2) If your bankroll can support it, play the second-biggest running game of whatever variant you choose. Let's say there's a poker room spreading from 1-2 to 10-20 NL, with the second-biggest game being 5-10. Let's say your bankroll can support playing at any of these. Should you play the 10-20? Probably not. Unless you can tell it's a really good game, stick to the 5-10. The 10-20 will suck up all the best players, leaving the 5-10 to be much softer. The second-biggest game of any variant is almost always much easier than the biggest one.
3) The overaggressive or overly loose player may be better than you. It is easy to dismiss loose, overly aggressive players as "morons" who make the game "great". Don't make that mistake. You may not understand their playstyle, and even if you do, it's possible you will have a hard time dealing with them. The best games feature ones where people are passive calling stations, and don't put you to the test very often. The only exception is a super-maniac who is absolutely spewing, to where you just need to wait for a spot to pick him off.
4) At limit games, beware of getting run over shorthanded. Limit games cost you more per round than NL games, due to the bigger blinds. For example, a 5-10 NL is roughly equivalent to a 30-60 or 40-80 limit holdem game, in terms of what you can expect to win or lose in a typical session. However, the blinds are far bigger at the limit game, which means you will get clobbered if you are too tight. In shorthanded limit games, there is no such thing as waiting for a good spot. You need to play a lot of hands, other than complete trash, and battle if you hit any piece of the flop. It plays completely different than full ring. If you don't have experience playing shorthanded limit poker, you're best off leaving the table when it gets short.
5) Go off the beaten path. If you're in a city with multiple cardrooms, try exploring and see if you can find better games. You can use apps like Poker Atlas and Bravo to see most (but not all) of the games running at local rooms. You can also call most of these rooms and ask them what's going. The less-visited venues also often have better games. For example, at the WSOP, don't automatically fall in love with playing the the WSOP cash area, the Bellagio, or the Aria. I'm not saying these will always be bad games, but you might want to look at what is offered at other rooms in the city. Don't go where the best players are likely to go.
6) Tight games aren't worth sticking around. If the game is incredibly tight, don't bother. You simply won't make much money from taking advantage of a game where the entire table doesn't want to chase or gamble, even if you adjust to be looser and attempt to exploit this. The only exception comes from #4 above, where you can still do well at an overly tight shorthanded limit game. However, in general, when it seems nobody is in the mood to put chips in the pot, it's time to switch tables or leave.
7) Don't ruin the vibe of the game. If one or more fish are being loud and acting crazy, act crazy along with them. If everyone wants to straddle, happily agree, even if you hate it. If you take bad beats, don't get overly emotional about it, and definitely don't act angry at other players. If you're fortunate enough to be at a game where everyone is having fun and being loose with money, don't ruin it.
8) The direct left of the worst player is often the best seat, but don't be obvious/obnoxious about it. Bad poker players are not necessarily morons. In fact, some are very smart and work in very difficult professions. You never want them to feel like the sucker, or that you're trying to gain edges on them. It can suck to have the worst player at the table to your left, to where he's either always re-raising with junk, or always cold-calling and leaving you confused out of position. If a seat opens up to his left, you can take it, but don't be obvious about it. Wait a little bit before grabbing the seat. If someone says they want it instead, at that point you can object if you have seniority at the table, but don't act like it's too important. Say something like, "Actually, I wanted to move there, I just got distracted and didn't mention it yet." If nobody is trying to take it, wait for maybe 10 minutes before moving. Preferably, move after you lose a hand, and say something like, "This seat no good anymore, I'll take that one", or "I can't win in this seat", and then move. This will prevent the bad player from realizing that you're moving to get position on him, and it won't ruin the vibe of the game. Remember, someone will probably get that seat soon enough, so it might as well be you. But there's ways to do it with subtlety, so you don't look like a predatory bumhunter. Don't chase the bad player around the table if he moves right after you. Accept seat-moving defeat at that point, and don't try to move again for awhile. In fact, sometimes the bad player will move back to give you position on him, so the problem will sometimes take care of itself anyway.
9) If you just busted a tournament in frustrating fashion, don't go play cash. It can be tempting to get "unstuck" a tournament buyin by moving to cash after busting a tournament. If it's a mundane bust, such as an early-event fail where nothing goes your way, then there's no harm jumping into cash. However, if you have just bubbled a tournament, just lost your big stack when running deep, just went out by losing a huge pot via a beat or cooler, or just went out 9th at a final table, it's probably time to take a break.
10) Think about where you are and when you're playing. An Omaha 8-or-better cash game probably isn't going to be that great when a lot of people are in town for a WSOP O8 event. In fact, the WSOP brings a lot of good players to Vegas whom you've never seen before, but are very skilled, so that's often the time the games in general are the worst. Your game selection goal should be to end up with the fewest likely good players at the table, and the most unskilled ones. A weekend like March Madness will bring a lot of recreational players to Vegas, for example. If you're not playing a limit holdem WSOP event, but you're a good limit holdem player, during the event is the best time to jump into the limit holdem cash games, as that's when the good players will be gone. Same with any other variant which is likely to attract away a lot of the better players. Think about who, in general, is likely to be at your cash table. Why are these people here, where did they come from, and how skilled are they likely to be? When you answer that to yourself, pick the times when the expected player group will be least skilled.