As a rule, I don't do ethnic accents, but here's my breakdown of how I roleplay each speech, for what it's worth:
Gnomes: Speaking rapidly, with a nasal accent. Lots of run-on sentences, jumping from topic to topic like a squirrel on crack. Tangent city, man...
Dwarves: Gruff, short sentences. Blunt, to the point, gravelly sort of voice. Stays on topic, shoots from the hip. Works from the starting assumption that everyone else has no idea what they're talking about.
Elves: Flowery sort of speech, few contractions if any. Evasive (keeping in mind the old Tolkein adage, and I'm paraphrasing here, "Never ask an elf, for he will say yes and no"), lilting speech.
Orcs: Gutteral, pronoun-challenged. As painful to say as it is to hear.
Halflings: Haven't used them very much, but they sound "normal," with perhaps just a bit of high-pitched tone. Polite, yet circumspect.