The TER ad section is down (for now, maybe forever).

A good way to advertise without the ad-specific board is to participate in discussions. When you *reply* to a message board thread you're allowed to include a link to your website or ad to include at the bottom of the post.

I've included one as a demo below.

I was hustling through the streets on my way to a meeting today and I wanted to check my phone. I "pulled over" to get out of the way of other pedestrians and stepped into a bus shelter. Many bus shelters in NYC and Boston have all-glass walls. Written on the glass with a marker was contact information for someone offering a place to go "4 fun" and so on. No TER Profile number. If that is too declasse (d'eclass'e - French) for you, maybe you can post ads on the actual MTA / MBTA rental space. You'd just have to keep your ad copy clean.  If you have ever seen the fashion ads on the bus shelters, you do NOT have to worry about posting suggestive photos.
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A while back, CDL suggested printing flyers and slipping them under the windshield wipers in parking lots and garages.  
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I suggested bus wraparounds.