Hi Ryan,
As Charles said,the best way is to use 1 click upsell feature(if you are using Clickbank).

If you use PayPal for payment processng,using "Rapid Action Profits" script is a good option.It supports two OTOs.

If you don't want to spend money,use the "OTO Goldmine" script.It is free and supports multiple upsells and downsells.
Thanks,
Vivek