Those confident in their willpower can get a credit card with 2% cash back rebate and just commit to donating the rebate. Methuselah Foundation's card only gives a 1% donation.
An interesting corollary: you can use that credit card to donate to the SIAI and still receive the cash back, effectively making your donation 2% larger, or hacking your willpower in a similar way as that described by Bo102010, but this time by reducing the cost of your donation.
In case you missed the notice at the SIAI Blog, the Singularity Institute's next Challenge Grant is now running - $100,000 of matching funds for all donations until February 28th, 2010. If you want your donation to be tax-deductible in the U.S. for 2009, donations postmarked by December 31st will be counted for this tax year.