The better way is to say "I always lie". If you're a compulsive liar you might just speak the truth once in a while on a rare occasion. But if you say you're always lie its a bit of a contraditiction. Atleast more so.
"Everything he says is a lie, so if he calls himself a liar he must be telling the truth, but everything he says is a lie, so it can't be the truth, but... ERROR! ERROR! DOES NOT COMPUTE!" </star trek>
heh. if you say you always lie, you're obviously lying. whether that's the only lie you ever told, or the only thing you've ever said that isn't a lie. that's not that big of a mind fuck. go watch labyrinth. that's more fun.