1) Estimate the amount of time it will take you to complete the project.
2) Double that estimate. This is Estimate A.
3) Determine the number of free hours you have left in the week.
4) Halve that estimate. This is Estimate B.
5) Figure out how much trouble you'll get into if you say no. Measure this in the number of hours it will take you to get out of the trouble. This is Estimate C.
5) If Estimate A is greater than Estimate B plus Estimate C, do not under any circumstances say yes. If Estimate A is less than Estimate B plus Estimate C, go back to Step 1 and reestimate everything, because you probably got your sums wrong.

Modifiers:
A) If the person asking you to complete the project is a cute member of the gender(s) to which you are normally attracted, expect a trap and double Estimate A.
B) If the person asking you to complete the project is a Professor or other person who might affect your grades, be very careful: these projects may have higher than estimated values for C.
C) If the project involves any kind of cash payment, add 20% to Estimate B.
D) If the project has the words "quick task," "should be easy," or "minor project" attached, double Estimate A. Any project to which those words can truthfully apply is not generally subcontracted.
E) If the project is for a journal, add 50% to Estimate A. These are never as easy as they seem.
F) Note that Estimate C may be subject to the Time Value of Trouble: that is to say, trouble that will show up tomorrow is less important than trouble that will show up today. On the other hand, trouble involving the aforementioned cute member of the gender(s) to which you are normally attracted may tend to be amusing enough to justify negative values for Estimate C.

Feel free to add your modifiers in the comments.