I've used fine sawdust mixed with a clear two part expoxy. The quick set stuff works well, both as a glue and a filler, and the color does not change a whole lot when hardens, It sands nicely, and will take a finish like tung oil, or a "wipe-on" poly. I usually pry the cracks open a bit using pointed toothpicks so that I get plenty of epoxy into the crack. Once the toothpicks are removed and the piece is wrapped with surgical tubing, or strips of old inner tube, the crack will close up nicely. When the epoxy has hardened, and is worked level with the wood, the repair can be nearly invisible, especially if you made a reasonable color match with the glue/sawdust mix in the first place.
I usually remove the old finish chemically, but always save the sawdust from any sanding, just in case I find cracks or other small voids that may need filling.
... not a pro, but it works for me