Caulked more gaps in the last month than I care to count, so here's what I wish someone had told me when I started. Cut the tip small. Smaller than you think. You can always snip more off, but a fat bead on a thin crack looks lumpy and takes forever to smooth out. Then run one continuous bead per section if you can, steady squeeze, steady pace, no stopping halfway or the joint will show it.
The real trick is tooling. You don't need a fancy tool, a wet finger works fine. Dip your finger in a cup of water, wipe the excess off on a rag between passes, and drag along the bead in one smooth motion. Wet finger, clean rag, repeat. If the caulk starts grabbing your finger instead of smoothing, it's skinning over and you're too late, so work in shorter sections next time.
Two more things worth the effort. Masking tape on both sides of the joint gives you a crisp line, but pull the tape while the caulk is still wet, not after it dries. And don't cheap out on the tube itself. The bargain stuff shrinks and cracks within a year and you'll just be redoing it. Use paintable latex inside on trim and around windows, and save the silicone for wet areas like tubs and sinks, since you can't paint over it.