Modern thinking is that sequences such as 1♣-1♥-2NT-3♥ are forcing - it is important to be able to find the right fit when game is on, and since many now play that the 2NT rebid is 18-19, finding the right game is more important than being able to play in 3♥ `.
At the same time, some people would be reluctant to pass many hands with a six card suit, even if they are below the traditional 6 points required for a response at the one level. (The reason for this is two-fold: passing these hands may well miss your best fit, and also it allows opponents an easy entry into the auction.)
You may notice that these two attitudes do not sit very happily together! On the one hand you respond light on the basis of a long suit, and on the other you are not even allowed to play in it if partner responds 2NT - not unlikely given how weak you are!
To combat this, I recommend the use of Puppet Signoff, involving a 3♣ enquiry (not to be confused with 3♣ checking for majors, a more popular convention, but not one I'd recommend). This has two benefits - you can sign off in your suit, and also have extra ways available to show other hands. The downside is that you lose 3♣ as a natural bid (don't underestimate this downside, though this can be accommodated to some extent).
When the auction starts 1♣ -1♦ -2NT, 3♣ asks partner to bid 3♦ , no matter what he has. [Incidentally, many people inaccurately describe this as a transfer to 3♦ . Technically it's a relay to 3♦ , as it doesn't show anything extra in diamonds. (ed)]
Then after 3♣ -3♦ :
Pass = wanting to play there.
3♥ /♠ = four card suits, denying five diamonds, unless you have no slam interest.
3NT = mild slam try with a natural 3♣ bid.
4♣ = stronger slam try, natural.
4♦ = what you fancy.
4♥/♠ = splinters, with five diamonds and four clubs.
If you don't use 3♣ , but directly bid something else, what does that show?
3♦ = 6+ diamonds with a slam try, no major (then major bids are cues.)
3♥/♠ = 4 card suits with at least five diamonds, and some slam interest.
4 level = splinters for diamonds (over which 4NT is to play, other bids are cues.)
Finally, for those that like puppet Stayman over a 2NT opening - where you bid suits you haven't got, in an effort to get the stronger hand playing the contract - you will notice that there is another chance to use that principle here. If the auction starts 1♣ -1♦ -2NT-3♣ -3♦ you can now reverse the meanings of 3♥ /♠ , so that you bid the suit that you haven't got. Over 3♥, partner can then bid 3♠ to deny spades, but show that he has four hearts, in case responder has both majors.
I've glossed over what happens when it starts 1minor-1Major-2NT. The principle of using 3♣ to sign off in your original suit is the same. Some of the other continuations are different - I leave you to devise your own, or drop me a line and I'll send you full details!