A friend asked me whether I used Garbage Stayman. I admitted to “Garbage Stayman” but not to “Trash Stayman.” Both conventions are artificial treatments after partner opens 1NT and responder bids 2 clubs planning to pass any further bid by opener. Although most players do not distinguish between the two conventions, Garbage Stayman by definition is limited to hands that have a 4-4-4-1 or 5-4-4-0 distribution. Trash Stayman extends the distribution to hands that are 4-4-3-2, and this significantly reduces the chances of an 8 card fit and at the same time substantially increases the overall risk of a bad result. Since I do not advocate the use of Trash Stayman, I will confine my remarks to pure old Garbage Stayman.
As the name Garbage implies, that is what your hand should represent. If your hand, combined with partner’s hand, has a significant chance of making 1NT, then you do not want to disturb the no trump contract to play what might be a 6 card suit at the 2 level (the dreaded 3325 distribution). If you have as much as 6 hcps, just pass. Partner will make 1NT or more about 65% of the time. On the other hand if you have 3 points or less, partner will make 1 NT only 7% of the time. So now we know that the biggest risk is to have too much! The best Garbage Stayman hand is when partner does not even have an entry to your hand.
Garbage Stayman is a defensive bid and not an offensive bid. You don’t care whether you get the bid or not, you just want to consume opponent's bidding space since they likely have more than 50% of the hcps. When responder passes opener’s Stayman response, the opponents now know that they are being "handled" on the deal, but by then there is no way they can recover and find their fit.
As a broader general rule, you should open 1NT as often as you can, since most partnerships have handcuffed themselves with unrealistic requirements in order to compete in the bidding over a 1NT opener (even without the intervention of Garbage Stayman). Even if the opponents hold 23 hcps, if they are at all divided between them, they are going to end up being spectators. When the Standard American no trump opening range becomes 14-17, remember that you heard it first from me! At the moment, only students of bridge probabilities seem understand this.
As a general rule, you shouldn't’t add any artificial bid to your repertoire unless it has more pluses than minuses. With Garbage Stayman the pluses for me are (i) the bid serves a need and I do not have an existing bid to do the job, (ii) the prerequisites of the bid are easy to recognize (4-4-4-1 or 5-4-4-0), (iii) the bid relies on stuff I already know (Stayman), so I don’t have to clutter my overstuffed head with new material and (iv) I don’t have to rely on partner to know what I am doing ( I have turned him/her into a puppet). The biggest minus, or maybe this is even a plus, you almost never have occasion to use it!