A good base will hold the interest of a critter on its own.
Anthony hit on the foundation for building a good lure. Take my Dig'r for instance. The base for it is actually my Beaver Meat Medley that is about 10 years old. After that amount of time it has gotten soft. So, I ground it with a fine plate and tested it to make sure the canines were still attracted to it. They were, so that started this formulation. I slowly built on it, adding one ingredient and testing, then another etc. I had to back up a few times to eliminate an ingredient that had no or little effect on the critters. What I was looking for at this point was to make the target animal stay and work the lure, rather than approach, smell and walk away. Each ingredient was only considered a successful addition if it increased the animal's response.
I use to punch a hole in the ground with a 1/2 in piece of rod, place my lure in this hole and sift dirt for about a 6 ft diameter circle around the hole. Then I would read and interpret the sign left. Since trail cams, this observation of the animal's response has become a much easier task. With a trail cam I can see what animals didn't even enter within the 6ft diameter circle I would have used before trail cams. The visual of those refusals taught me a lot that I missed with the sifted dirt.