|twitter_spam, originally uploaded by ecblogger.|
I don't mind receiving plugs once in awhile. However, if I open the sender's Twitter feed and all I see is the same message sent to multiple folks, I perceive that as spamming.
Here's what I suggest to avoid getting your Twitter account or you tagged as a spammer:
1. Use direct message.
If the person you are sending a message to is also a follower, best to send them a direct message.
Then add a follow-up message that you are doing it as a one-time plug and disclose if needed (like you are joining a contest or hired to send such one-time message).
By personalizing it, the person can even do you a favor of sharing (or re-tweeting) your message, among others. You might even get a prospective client referral.