Try this amazing (and very funny) Science Fiction Plot Generator and you'll never get stuck on the plotting again.
Speaking of which: Do you sometimes get the feeling "I've heard this plot before"? Chances are, you have. When all else fails, the writer can always fall back on clichés and plain cheating.
For further studies in Generic Science Fiction Plotting, read Nick Lowe's hilarious essay "The Well-Tempered Plot Device".