My favorite isolation exercise for triceps is a lying overhead extension (db or bb). At the starting position, weight should be on or near the floor and upper arms should be in an overhead position parallel to the floor (or as close as shoulder flexibility allows). Keeping the upper arms in tight into your head, raise the weight to a finishing position where arms are straight and parallel to the floor. Hold for a 1 second pause then lower and repeat. For me, these tend to require lighter weight, but when you hit the sweet spot, they really sneak up on you. A nice finisher.