A lot of good advice, and some stuff that may not work universally has been given here...so I wont feel so bad if my advice falls flat.
For some people, "Be yourself", or "Be natural" falls flat because it is our nature to think about things, and to try to take an optimal action. Sometimes, there isn't sufficient time for that, but there often is. So for people like me, I say:
For important decisions:
1) Have a decision ready. You can keep thinking, and feeling tings out, but have a decision ready, in case you are forced to move. When thinking, always do it in the context of how that would change your decision.
2) Know when you need to take action. This is usually a gut-level feeling. But be on the lookout for people who try to force decisions from you earlier than you'd like.
3) Try to think of the consequences of your actions on several times scales...immediate, short term, medium turn. long term, and forever terms.
4) Try to take into account the needs of all the people affected by the course of action you choose. However, ultimately, your decision is yours, and it is OK to be a little selfish.
For spontaneity, of course, the above advice falls flat. I may not be the best person to give advice for this but:
1) Be playful. Amuse yourself, entertain yourself, make sure you are engaged.
2) Share your playfulness with others.