一番よく使われている表現はThanks for the heads upです。
この2つの表現は同じ意味を持っていて、現在や未来のある事実や状況に対して教えてくれた相手に感謝を表す時に使う表現です。 "Thanks for the heads up"がより一般的に口語体で使われる表現です。 "Thanks for a heads up"はあまり使われませんが文脈によって特定の状況下で使われる表現です。
Thanks for the heads-up, Earl.
Okay, thanks for the heads-up about the meeting.
Thanks for the heads-up last night, Janet. It gave me some time to plan my night.
Thanks for the heads up, Artie. See you at the softball game on Friday!
Sorry I missed your call, Mr. Coleman, but thanks for the heads-up.
Although it could be too late to change plans, thanks for the heads up.
Thanks for the heads up. I haven't had time to fix myself up.
Thanks for the heads up on the credit card usage--I didn't know I had spent so much!
Thanks for the heads-up, by the way.
Thanks for the heads-up on the eyewitness who saw the redhead.
"Thanks for the heads up," said the man, after his friend told him that he had jury duty the following week.
Thanks for giving me a heads up on this project.
Okay,** thanks for a heads** up regarding the house inspection.
Thanks for giving me a heads up last night, Janet.
Can I give you a heads up on what she is going to ask you?
Sorry, Steve, you didn't give me a heads up on this matter earlier.
Thank you for giving my mom a heads up about the family gathering in advance. I appreciate it.
I gave you a heads up but you didn't listen and jumped in the frozen lake anyway.
Thanks for a heads-up on any future plans we might make.