I've been using a "gripple" to tighten some old loose barb wire fences. If the ends and braces are tight, I cut about a 2-3 ft section of barb wire out of the middle of the slack area and use 2 gripples to insert a piece of smooth 9 or 11 gauge brace wire and pull the smooth wire tight with the gripple tool. By tightening it in the middle you have to pull a minimum number of barbs thru the clips. The roll type tighteners don't seem to work to well for me on heavy Red Brand barb wire. Old wire tends to break there.

If the ends are the problem, I use a new piece of 9 or 11 ga brace wire wrapped around the post, stapled and twisted with about 3 ft of and end loose. Then use a large gripple to attach the end of the new 9 or 11 ga to the barb wire and tighten by pulling the smooth wire thru the gripple with their tool.
To me this is easier than using the older type fence puller.

Here's a link to the Gripple website with some interesting little animations showing how to use them with barb wire. click on the American flag, Fencing and Barb wire.

Jim at Dawn