RESPONDING TO WEAK PREEMPTS
Responding to weak preepmpts is almost the opposite of responding to NT. Instead of strength, there is weakness. Instead of balanced distribution, there is length. Your partner's preempts kill the bidding space of the people sitting on either side of you. Unfortunately, it does the same thing to your bidding space. The advantage you have over your opponents when your partner preempts is that you have constructive information about your partner's hand; your opponents know about your partner's hand, but they don't know about one another's hands.
High level preempts such as those at the 3 and 4 levels are difficult to respond to. You really need to have a hand that is almost self-supporting to switch to a new suit as you can count on very little help from your partner. As a general guideline, you partner can only pass your responding bid (assuming his RHO passes) if he has 3-card or longer support for your suit. Even with a complete bust, he will rebid his suit and tell you to shut up.
In responding to partner's weak two bid, you have some options available to you that aren't available when responding to higher preempts. First of all, if you have three or more of your partner's suit and reasonable (a little over opening) HCP, raise directly to three. This invites a raise to the four level if partner is maximum. Note that this provides little benefit in the minors since game in a minor is reached at the 5 level, and you are only providing enough power for a raise to the 4 level, mostly because of your extra trump. Note, however, that if your RHO overcalls your partner, you can and should raise to three on any hand with 3-card support, to four on any hand with 4-card support. You are safe to a level that requires that you win the same number of tricks as you have trump between your hand and your partner's.
Ogust
One popular conventions for handling weak two openings by partner is Ogust. With any hand that is invitational to game (you have at least 2-card support for partner's suit and your points plus partner's maximum would be right for game or better), bid 2NT (assuming no interference by opponents). This asks partner to bid a suit at the three level and, in doing so, tell you two things: the range of his points and the quality of his suit. Here are the meanings of the responses (all of which must be alerted):
With the information from these responses, you should have no trouble placing the contract at the appropriate level. Note that if the suit is diamonds, and you are unwilling to play in 4D or higher unless your partner has 9+ HCP and a good suit, don't bid Ogust. In that situation, he could bid 3H and force you higher than you want to go. This should be the only situation in which a 2NT Ogust bid is a risk. Remember that 2C is a reserved bid, so Ogust never applies to it. Furthermore, in hearts, you can always stop at the 3 level if partner doesn't have the best possible response for you, and you will never bid Ogust if a 3S response is not enough to go higher. Never bid Ogust if a 3-level contract is the highest you will go under any conditions.