How should I title my "thank you" e-mail after technical phone
interview? Is just "thank you" itself enough?
Is there some accepted practice?
When I send a follow-up email, the subject often says simply "Thank You" or even just "Thanks".
I know of no "accepted practice" here, but I don't think the Subject line for simple thank you emails is overly important, anyway. Since it's coming from you, and the recipient presumably remembers your interview, they are sure to read it anyway. Capturing the potential reader's attention is the point of the Subject, and that really doesn't apply here. The contents are where you want to spend most of your time and energy.
Express your thanks. Reflect on what you heard. Expand on any points you think will put you in a better light. Show why you fit so well for the position. Show enthusiasm. Ask the interviewer about the next steps and the timing if appropriate.
Make sure there are no typos or grammatical errors, and in particular make sure the name is correct.