Its very simple to tie, just a termination knot leaving a long tail, then basically use it as a choker. If you get tired of sitting on your spikes, you can hang on it, plus if you've limbed out the tree, or just coming down a spar, instead of spiking down you can just descend, dig your spikes in, flip off w/ your lanyard and tug on the tail to slack it so that you can pull it down to you and repeat. This works really good as a second tie in while you're chunking down big wood or the spar. I think it would work well in your situation if you're not yet used to your spikes, gives you a break from standing on them when theres no branches to hang your climb line on.
Something else that might help you, is pointing your spikes at the tree which sounds obvious of course, but I mean like toward the center of the trunk (perpindicular), not pointing at the ground. The more towards the ground they are pointing, the more you're likely to slip. I've slipped many times and the spikes always catch, but it can be a bit unnerving doing a drop and sliding down till they catch. It might help to remember to spread your knees a little, that will point the spike into the wood and less toward the ground.
Hope this helps.
