[url=http://www.springhole.net/writing/write-better-assassins.htm]This[/url] is an excellent guide on the matter. But to summarize what undoubtedly is a long article and an even longer leap down the rabbit-hole (this person has tons of interesting pieces):

[list][*]Good assassins blend in. Uniforms are a bad idea. Also a bad idea is to look like you're up to no good; so no hoods, no masks, no knives strapped to your boots and bracers.
[list][*]Hoods are designed for inclement weather, so wear them only when it's raining, snowing, much too hot or much too cold.
[*]It may seem like a good idea to cover your face, but wearing a mask lets people know that you're trying to cover your face. Immediately it raises suspicion. If you must wear a mask, have a good alias; maybe you're a leper, or a burn victim. Maybe you're attending a masquerade.[/list]
[*]Use generic weaponry and equipment. A super-speshul one-of-a-kind hundred-year-old ultra mega awesome sword just makes you easier to identify.
[*]Fight to win, not to look cool. This means no backflips. It also means no long gloating speeches.
[*]Throwing knives don't actually kill people; at best they're distractions. Bows aren't very good for long-distance sniping; they were fired in volleys, and archers learned how to shoot at [i]distances[/i], not at precise targets. (Fifty meters, a hundred meters and so on.) If you really want to be a medieval sniper then you'll want a decent crossbow instead.
[*]Most assassins and hitmen are hired to whack cheating spouses, asshole bosses, and other small-time lowlifes. Don't always play an Agent 47 or a Corvo Attano, killing enormously powerful fat-cat politicians.
[*]Reputations are a [b]bad[/b] thing. Avoid having one.[/list]