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putting it in the freezer to stiffen up the meat helps alot. I always use a slicer. I had an old one that was mostly made of plastic but just recently got one off craigslist that was all metal pretty cheep. the electric slicer makes it sooooo much easier.
I'm usually still in the thaw process when I go to slice, so it's softer on the outside than in the center. Use a freshly straightened chef's knife (I put a steel to every knife, every time I start a new cutting/carving project). The deeper blade of a chef's knife helps in making straighter cuts through thick pieces of meat. especially when partially froxen...a carving knife tends to wander more in my hands.
I've seen those jerky slicing boards, but never felt it was necessary. I'll have some slices which are a bit thicker and they go into my known hot spots in the smoker, so they'll be dried about the same time as the average slices. Inconsistencies in thickness are not any reason for concern if you use them to your advantage. Also, inconsistent thickness in jerky gives you a slightly varied chew and texture...you never know what you'll like the best if you don't try new things. I made some beef-steak style jerky a few months back (3/8" thick slices, cut free-hand) and I thought that it was the best batch of whole muscle jerky I had made thus far. When you hefted a slice of that jerky and took a bite, you knew it was going to last.
Do you have a slicer or are you slicing them by hand? A meat slicer is the way to go and you can get them for a pretty descent price. Like the others said if it is slightly frozen it will slice like a dream.