I just had a new clutch put in less than a year ago and now it slips out of gear while driving down the road. What could be the cause of this without having to put in another new clutch?

That's not the clutches fault! .. IF it slips out of 'one gear' only .. Suspect 'internal' transmission problem!

Your shifter may have worn bushings . your shift cables may need adjusting. They are probably not allowing it to go into gear properly.