What's the best thing to do when your back goes into a spasm?

Back spasm.: When the muscles of the back go into a painful spasm, benefit would come from anti-inflammatories initially, then perhaps prescription muscle relaxers. Light activity is the rule, not bed rest.

Back spasm: Other than rest, applications of cold alternating with heat, firm mattress, antiinflammatories like Advil, (ibuprofen)if you can take them, applying bengay liniment, and having your physician prescribe a muscle relaxant like flexeril if he/she agrees. Also applying a lidoderm analgesic patch over the painful area and wearing a low back brace may be recommended by your MD.