Buy smaller than feels impressive. First-timers reliably overbuy. Something in the range of an inch and a quarter to an inch and a half across is where most people should start. If it turns out to be too small, that is a good problem and a cheap one to fix.
Match the base to the O-ring, not by eye. Harnesses come with interchangeable rings for a reason. A base too small slips through mid-scene, a base too large will not seat at all. Both are avoidable by checking two numbers before you order.
Dual-density silicone is worth the difference. A firm inner core with a softer outer layer gives you something that holds its shape in a harness but still yields against the body. Single-density silicone does one or the other, not both.
Curve toward the receiver's front. Load the dildo into the harness with the curve angled up, not down. It is the most common setup error and it is the difference between finding the prostate and missing it entirely.
Warm up first, every time. Fingers or a small plug, several minutes, no rushing. This matters more than any product on this page and it is free.
Never anything with a numbing agent. Pain is information. Numbing it removes the only signal telling you to slow down or stop.
