Having watched a lot of games in my years and having gotten a lot of feedback (and strokes) about my roleplaying ability, I am very confident putting myself in the top 10%.
My characters have distinctive voices, attitudes, desires, and habits. I often know about their families, friends, and acquaintences, though the other players may not find this out until a moment where it is appropriate. Sometimes players are puzzled over the actions of my character until they learn a bit of backstory about me, but again this does not come out all at once, but slowly over the course of a campaign.
Sometimes I use small props (nothing big, nothing that would get in the way of the game) that help solidify my character at the table -- I had a PI who always carried a pocket notebook and a mechanical pencil, for example, so I always had these at the table for taking my notes. Equally my characters often will purchase "appropriate" items in town, rather than necessarily the "best" items -- again, my PI was a bit of a slob so I always paid less for his clothing, often buying it used, but I paid more for his booze, as that was important to him, and he always had a bottle with him.
Small things go a long way towards creating a memorable character. Sometimes it is a turn of phrase, a set of gestures, or simply always asking for purpleberry tarts in the market. Instead of saying, "Who was your character again?", other players tend to say, "Oh yeah, Charlie Eastway always gets that in this town. Kinda weird, but that's Charlie."