I’m one of the fortunates who has had lucid dreams for a while without even thinking about it at all. For many years ago i knew once in a while that i was in a dream, and therefore could control what was happening. I have problems getting lucid at will but that’s not my point here. What the big problem is, is once i realise that i’m in a dream, i tend to decide what i wanna do and right after it i wake up! I realise “Yeah, i know i’m dreaming, i can do anything!” and just after it i wake up. It’s kind of a mental barrier i suppose, where the thing i really want keeps slipping between my fingers because i don’t want it to. It’s like when you want to picture a face, say of a person who’s really good looking, but you just can’t. You can see everyone elses face quite clear, but not that face, and i think that’s almost the same as my problem. When i finally get what i want, the lucid dream, i can’t hold on to it… i just wake up! Any suggestions on what i can do to prevent this?
Here are a few techniques to practice once you become aware that you are dreaming, that will fasten you into the dream realm and prevent you from awakening:
- Rub your hands together.
(and/or)
- Fasten onto a nearby object and rub it, feeling the detailed sensation of touch.
(and/or)
- Say out loud, “Increase REM length, NOW.”
(and/or)
- Just chill and relax.
Basically, everything darxide said will work 
Trying to stay relaxed helps me. What I do is say, “This is a dream…so what.” To try to calm me down, when I’m calm I start doing things slowly. Then I do it until I know I won’t wake up from excitement. Hope this helps.
Does dream spinning (spinning round in the dream) help stabilise the dream & keep you in it, or do you just use this for changing scenery/ changing the dream, or escaping mutant star goats that are trying to eat you (umm, for escaping nightmares, in other words 
)?
Yes, dream spinning helps stabilise the dream but when you do this you will find that the dream scenary changes, well, for some people anyway.
Here’s another tip that works for me…
When you find your dreamworld fading away try looking down at your chest. If there is writing on your shirt focus on it. Because it is your body you are looking at and its connected directly to your head it will be harder to fade away and the more you focus on it you will see the scenary beneath you and around you come back to normal. Once you start seeing the ground beneath you come back you can try focusing on that then the rest of the scenary.
I came up with this technique myself in a dream I had, it may be a lot like the looking at your hands technique but the difference is your looking at something closer to you with less scenary around it so the mind has less work to do.
I have this problem as well. I think of a million things I want to do before I go to sleep or after I wake up, but once I’m actually lucid, I either can’t do what I wanted, or I can’t remember to do it!
The last time this happened to me I remembered exactly what I wanted to do. I wanted to make a book of predictions for the hurricane season appear in front of me so I could read it. I couldn’t do it, so instead, as to not waste the dream I started flying.
While I was flying, I started to lose lucidity. So I spun around. As the previous poster said, the dream image changed and I soon woke up.
I find dream spinning works the best to answer your original question. But I find it helpful to have ONE thing I like to do in dreams as a backup plan. That is flying for me. But at least that way, if you can’t remember specifics or can’t make something happen, the dream doesn’t just go to waste and fade away.