Well, I see two ways here. 1. Read the post, think it over and write a good comment. 2. If for example English is not your native language (as it is in my case) or you have found a do-follow page with a good PR but the post you want to comment on is far from your sphere of interests and you don't know what to write, just find a forum where people discuss a similar question and get ideas (or a comment;-) ) from there. It works well for all kinds of review posts, i.e. gadget reviews, movies reviews, etc.
I think that the best way is to read to blog post and the comment should be relevant to the post and not something meaningless or advertisement of your site or services.
Do you guys generally include a link with anchor text in say a signature? I find that a lot of these dofollow blogs frown on stuffing keywords in the "name" field.
Look around at what other comments are on the the site. If they look like the name is keyworded anchor text for the name then they probably will allow yours providing your actual comment text is more than a few words and on topic with the blog. If you don't see other comments that appear to have keyworded anchor text for the name, then they may not allow it. In that case I sometimes use a name that could be a keyword and less likely for others to think so. If it is nofollow I don't bother. If it is a low pr I don't bother, but if it is something with decent pr and not many links on the page, then I will take a little more time to think about it, and look around the site to see what's going on, but only if it is really worth it.
1. Never post a comment on an OLD post with high PR. Go for newer posts, where author is still actively moderating comments and is likely to participate in discussion. 2. Don't use keywords if the blog doesn't have keywordluv. 3. Read the post thoroughly and if you don't understand the content, don't force yourself to make a comment.