Well, just make 'em work like monsters. Then, add in XP to each combat equal to the XP of the hirelings, and don't award it to the single PC.
For example, you have 1 1st level PC with 2 1st level hirelings. An average combat would have an XP budget of 300 XP, but you'd only award the PC 100.
Alternately (and sorry for getting a bit OT here), you could run just one PC, no hirelings, but survivability may be a problem (especially if the player is a striker or controller). Leaders and defenders would be reasonably survivable, but a leader wouldn't be able to lead himself, so a defender would probably be best; I'd suggest a paladin due to it's healing abilities. Going by the above example, an average encounter for a lone level 1 would have an XP budget of 100, all of which would be awarded to the PC.