How to be Dash from the Incredibles....

Have your Mom go to the store and realize that since it's not Halloween, they do not have any spiderman fabric.
When your Mom asks what would you like to be instead, make sure to shoot webs to reiterate your point that Spiderman really is the only superhero worth being.
When Mom reminds you of this...

Remember that it is not fun to pick to said Spiderman costume out of your butt all day. It's kind of annoying.

Although, eating candy does let you momentarily forget about previous annoyance.
Ultimately decide that Mom won't let you eat enough candy to compensate for the annoyance ALL day and that being Dash might be fun.
Once your Mom makes most of your costume remember to get really excited for arms in it this time. (Mom had a tough time with those last time).
Remember to sing the You're the Best Mommy Song while she sews. I find that Mom generally forgets that I was pulling the spool off of the sewing machine and jumping up and down over the pattern pieces.
The song really seems to help.
In addition to being fast, remember to arrive fashionably late to your friend's birthday party. Jumping on patterns and pulling spools off of the sewing machine does seem to add some additional sewing time.
See above reference to singing the Mommy song.
But most importantly remember to look cute and extremely appreciative, it really makes Mommy's day. She then stops ranting about, (I don't know why I do this stuff for you, you need to be a good listener if I'm going to be able to finish this. etc. etc.)
See above referenced Mommy song.