My hand is decently small, and it's very difficult for me to play a series of octaves for a long time. The piece I'm playing has a bunch of octaves for a couple measures, and I'm wondering if there's anything I can do (other than practice the song multiple times)
Stretching out a hand is stressful on the abductors to begin with and pulls it in multiple directions. Compound that with flexing or extending and you will create much tension. All tension comes from trying to use two muscles at the same time to move one bone in two directions.
Make sure you are playing with the weight of the arm or gravity. Between each drop of the arm, make sure (if there is time) to allow your hand do adduct. Don't hold an abduction. Especially if there is flexion involved. You may be able to use momentum from rotation the pronator and supinator muscles to spread the hand out rather than abducting.
If you can at least reach the octave, then the tension isn't from the stretch alone.