You should feel pretty free to overcall at the one level pretty much any time you have 9 (even a good 8) or more points and a suit of at least five cards.  Your partner will not expect more than this unless they hear more from you. 

When you overcall at the two level you should have not only the five card suit that you need for any overcall but a good suit - containing at least one, preferably two honour cards and about the same strength as for an opening hand.

Suit

Think about where your long suit comes in the suit ranking.  If you are overcalling in spades, which will push the opponents to the next level if they want to bid a suit, and allow your partner to show support above any bid that your left hand opponent makes without going up a level, you can afford to do so more freely, and with weaker hands, than if you are overcalling a lower-ranking suit, say 1♦ over a 1♣ opening, which doesn’t have either of those advantages.

Vulnerability

When you have been warned by the opening bid that at least one of your opponents has quite a good hand the risks of entering the auction are higher so you should only do so with better hands if you are vulnerable or you might find yourself conceding a big penalty score.

Jump overcalls, made as pre-emptive bids, should take into account all of these factors, and also whether or not your partner has already passed. If they have (i.e. you are third to bid) you have an extra useful piece of knowledge to apply to the tactical decision. Weak, distributional hands can afford to be more aggressive in this situation, especially when non-vulnerable because your opponents are known to have the stronger hands between them. If you are making a bid that jumps a level (i.e. you could have bid your suit at a lower level) then the rules of the game demand that you should say ‘stop’ before you bid or show the ‘stop’ card if using a bidding box.  This is to enforce a wait of around 10 seconds before the subsequent bid so that the tempo at which the next player bids doesn’t reveal information about the strength of their hand. This rule isn’t applied when playing online and you can choose to waive it when playing amongst friends, but you would always have to do it if playing in person in competitions.

Takeout doubles

A takeout double should have shortage in the suit your opponents have bid, the strength of an opening hand and good support for the other three suits.  Think SOS - help me partner - bid something!:

S - shortage

O - opening values

S - support for the other three suits

So a ‘perfect’ takeout double when your right hand opponent has opened, say 1♦ might look something like this:

♠ A T 8 7

♥ K J 7 4

♦ 5

♣ K Q 9 4

Of course, real life is not always so tidy and your decisions about when to make a takeout double will often be more marginal.  If your right hand opponent has opened 1♦ and you have this hand:

♠ A Q 7 

♥ K J 7 4

♦ 8 5 3

♣ K J 4

It doesn’t satisfy all of the SOS criteria - you don’t have shortage in the suit your opponents have bid - but you do really want to compete.  You have 14 points and you are happy to play in any of the other three suits.  If you don’t take this opportunity to tell your partner that you have quite a good hand you may not get another chance.  

The one situation in which you really don’t want to make a takeout double is when you have good cards in the suit that your opponents have bid. For example, if your right hand opponent opens 1♥ and you have this hand:

♠ J 7 3

♥ A Q T 8

♦ Q 5 3

♣ K 6 4

You are happy for them to try to play in hearts - you would only be helping them by forcing your partner to bid and rescue your opponents from what could be quite a tricky situation. An overcall makes the auction a little more difficult for your opponents and gives information to your partner so it’s a good tactic to use when you can. A good tactician, however, will take account of the circumstances in deciding when to act.