Ah, the Shotgun Approach, or the Throw it against the wall and see what sticks approach. Both are effective if the product and/or service warrants that type of marketing.
My traditional career revolves around advertising and marketing, that's my bread and butter. I've been involved in a host of mail campaigns since 1990. I see what works and what doesn't based on industry.
For those who do not qualify for the Shotgun approach, the next option is to focus on those trade magazines. These are double optin subscribers and a very targeted audience. You can't get much more targeted than an industry magazine.
I always recommend full page right facing ads if available and if budget permits. I recommend at least running the ad in six issues (if monthly). Use custom URIs and email addresses for tracking (this is a must).
When coming at this from the Shotgun approach, there is a limited trust factor. And, if you do them bulk mail (third class), most are going into the bin, especially with larger companies that have mailrooms. Use a first class live stamp for maximum sub-conscious impact.
Definitely a great concept and one that I promote to my local clients. We've taken it a step further and in some instances have included a detachable business card, refrigerator magnet, calendar, etc. This takes the effectiveness of the campaign to a new level. Almost everyone will take a magnetic fridge calendar and put it up. Especially if it is usable and doesn't contain an over abundance of marketing lingo. These are great for restaurants or anything where the user references it on a regular basis.
A sure way to get your recipients' attention is to go the extra mile and do something totally different from what everyone else is doing. A plastic envelope. Frosted with tone on tone colors. High quality materials that cause oohs and aahs from the recipient. First impressions are lasting ones. Do it right the first time around! ;)
Of course all of the above is relative to the product and/or service. If the product is high dollar, you'll spend the extra bucks to produce a high impact mail campaign.
If the product and/or service is low dollar, you'll opt for the inexpensive post card, door hanger type campaigns.
For high dollar products, you can't use the inexpensive approach to marketing your products if you expect to attract the high quality buyer with money burning a hole in their pocket! :)