Our typical letter is a printed, personalized letter and we usually have someone local stuff the envelopes and handwrite the addresses on the envelopes. We've done postcards (our response was too low to make this worthwhile again) printed business letters (2-3% response rate), printed personalized (usually +5%), and handwritten letters - either handwritten or looks handwritten (+5%).
It's usually important to cover the following:
1. who you are - this distinguishes you from the others who send mail and allows you to connect more effectively with people you'll bond with. (this is pretty hard to convey on a post card)
2. why you feel their property is a good fit for you
3. call to action - there should be more than one call to action in a well written letter and you should always give multiple ways they can contact you i.e. your address is at the top of the letter, you phone number on the letter, email address. Some people are nervous about making the call to you so every now and then you'll get a letter back or an email from a potential seller.
Your letter is a sales piece at the end of the day. You are selling the owner on connecting with you. You should put an enormous amount of thought and effort into your sales pieces and follow up so that you can maximize the money and time you'll be spending on this. We probably spent $10,000 on direct mail in the last 12 months but we've bought 6 properties through that effort and have 3 more currently under contract with a handful of others that are close to contract. It's money well spent when done correctly.