Mirror extensions.
Not to disagree with daddydarko (we just saw Donnie Darko, by the way, it was great), but I didn't bother changing grips on our last five bikes and it was aways fine.
Mirrors that show me myself don't help, however, and that's what the stock Spyder mirror does. I bought the mirror extensions actually before my Spyder was delivered in March of 2008. I rode it one day with the stock mirrors and it made me crazy. Maybe I just check behind me too often?!
To be fair, I intend to put Kuryakyn grips on, and I have recently added the BRP weights, Crampbusters, and 3 inch NMN handlebar risers. I'm just saying that I put the mirror extensions as the #1 or #2 mod (along with highway pegs) and grips, etc, are coming in around twelfth place on the list...