As your ASP applications get more advanced and grow in size, little bugs start becoming very difficult to catch and fix! It is important to grow your debugging skills while you grow your coding skills! Here are some tips that will hopefully aid you in your debugging:
Response.Write's like there's no tomorrow! Don't be afraid to print out the contents of your variables at different times during the execution of the script. This helps tremendously! Also, think logically. Step through your code one line at a time, perhaps writing down on a piece of paper the current states of your variables, for loops, conditionals, etc.
Comment out code that you think might be causing an error. If you just comment out a line at a time until the error or problem goes away, simple induction tells you that the error last commented before the error disappeared was the error-causing line!
Simplify, man! If there is a ton of code, but you know the error or bug is within a small subsection of the rest of the source, cut the rest of the code out of the page for the time being just to clear everything up, removing the clutter. You can paste the non-offending code back in once you have trapped the error.
Here is a related article, that may help!