There are 2 airline point purchase promotions currently going on now:
The first is with US Airways where you can purchase points and will get a bonus up to 100% depending on how many miles you purchase up to 50k miles. Promotion ends May 25, 2014.
The second is with Alaska Airlines where you can purchase points and get up to a 35% bonus depending on how many miles you purchase up to 40k miles. Promotion ends May 25, 2014.
To me these are usually good to top off your account to get to the next award threshold. Also remember that these are processed through points.com so you won’t earn an “airline” points bonus. I suggest using a 2x card or any other card that gives you a flat rate for all purchases.