I just cut mine with a big, sharp steak knife. I make soap on a small scale (2.5 lb batches at a time), so this works out fine for me. If I was large scale, I would get a Tank cutter. With a knife, it takes practise, but I've actually become fairly good at it over time (not to say I don't mess up now and then :wink: ). I save the crookedest bars for myself.
First, I measure out how big I want my bars to be with a ruler, and then I mark out little notches on both sides of the soap so I can line my knife up on the notches to cut to size. Then, with one hand on the handle of the knife and the other on the opposite end, I press down as straight and fast as I can. I find that the faster I press down, the straighter the cut.
I used to cut by hand until I decided slab molds were for me. I can still do swirls and some neato stuff with the slab, so it's good enough for me. I had my husband cut lexan dividers for me and like IrishLass, I soap 2.5 lb at a time. Small stuff. I can't write straight on a line, so cutting straight is even a bigger challenge.
I'm like IrishLass: I score the soap at 1 inch intervals with a ruler, then use a big chef's knife. The only difference is I heat a big pot of water to just below boiling and hold the blade of the knife in it between cuts to both heat the blade and to keep it clean. With practice I've gotten better at making straight cuts, but I still want a tank-style wire cutter for PERFECT cuts.