I've been using this technique for quite a while now, and i forgot what first sparked the idea (it was probably from someone here on this site, my bad if i'm repeating something).
Once you've become lucid, quite often you might experience a rush of excitement, which will often cause a new lucid dreaming to wake up pretty much straight away. People suggest spinning and rubbing your hands together, which seem to work well and is great in overwhelming situations when the dream is on the extreme verge of collapsing. My method is used mainly when the dream is moderately unstable (you have roughly 10-15secs before you would wake up if you did nothing), or to increase clarity.
Once i've become lucid, i'll immediately sit down cross legged (depending on the stability i might move somewhere comfortable). I'll put my hands in the usual position when i meditate in RL, but not close my eyes (don't do that you might wake up). I un-focus my eyes though, so that i can still see my environment, but not focus on anything in particular. Once i've gotten into position, i'll take in one really big breath, and usually that will automatically calm my body down, enough to eliminate the chance of the dream ending or after 1 or 2 more breaths. Once i've calmed myself down, I'll modify my breathing so that my focus is on the idea of me 'breathing in' the dream. Its difficult to explain the feeling, but its like i'm no longer so much an intruder to the dream, but i've mixed myself with the dream, like i'm just as much a part of the dream as the dirt i'm sitting on. I'll carry this on ranging from 30secs to up to 5mins. The more i 'breath in' the dream, the more stable it becomes and the clarity also increases.
Its not only for when the dream is unstable, often i'll just do it for the sake of it, and its never had a negative effect only ever seems to increase my awareness.
