Disclaimer: This is the result of my idle thoughts on a Sunday night, and thus is not scientific in any way. No actual research was done.
Although some people seem to think that a newsreader is the be-all-end-all way to read blogs, there's really quite a few approaches. Site organization and navigation structure can aid these different reader modes, though the design decisions may not always be easy. Facilitating one may hinder another; trade-offs have to be made. To be more specific, here are a few reader "profiles" and the things that impact them:
This person is subscribed to your RSS/Atom feed, and is aware of every single entry you publish (reading it is a different matter). If you provide a full-content feed and they choose to read it in their aggregator, then there isn't much that needs to be done, beyond making sure that your feed does in fact accurately reflect your site (e.g. for a while I had forgotten to include all of my MTMacro definitions in my feed template). If they end up reading the entry in their browser, it is best to minimize the extraneous clutter that surrounds the entry text. This can be as simple as making sure that each entry has its own page, but it may also involve removing any sidebars, headers or footers that aren't really relevant to that entry.
Since the magic that is RSS/Atom hasn't reached all corners of the earth, some loyal readers may still resort to visiting your site periodically, to see what new things you have posted. Or perhaps you don't provide a feed so readers have no choice but to check the old-fashioned way, via bookmarks. Such readers still visit often and thus are familiar with your site's organization; therefore the previous clutter minimization strategy still applies. However, it is also important to make it easy to see which entries are new. Basic things like making sure you have a different color for visited links matter. The traditional reverse-chronological sorting can be somewhat annoying, but assuming that that the last entry that the reader remembers hasn't fallen off the front page, it should be a matter of scrolling down to it and then reading one's way back up. If in fact there have been so many updates since the last visit that all front page entries are new, then more aid is required. The calendar is of some benefit, although it does require the user to remember the approximate date of their last visit. Providing a way to navigate from entry to entry also helps. Perhaps the best solution is to convince such readers to subscribe to your feed, to make both your lives easier.
Depending on which pundit you listen to, linking is the essence of blogs. When a visitor first comes across your site as a result of a cross-site link, they are placed in a pretty unfamiliar environment (depending on how much you deviate from standard templates). The link that induced them visit your site may have provided some context, but that may still not be enough. A sidebar that points to the "surroundings" of this entry (posts in the same category or close chronologically) may help a reader who does not have the background knowledge of a frequent one.
Someone coming across one of your entries as the result of a search may be considered a subset of the previous reader type. The key difference is that they have nearly zero context, beyond (possibly) the snippet that was in the search result listing. Having individual entry pages is key (assuming the search engine is clever enough to favor those over time or category archives). There are ways to make the searcher's life easier, but in my experience they are of limited usefulness. Search engines themselves strive to solve this problem as well (e.g. Google's cache with keyword highlighting), and in the meantime "surroundings" suggestions apply.
Perhaps this is the procrastinator in me speaking, but I periodically come across a blog that seems interesting and focused enough that I want to read it from beginning to end (or end to beginning if I'm feeling adventurous and/or want more timely information first). Facilitating this is as simple as providing previous/next links on the individual entry pages, but it's surprising how some default templates don't support this behavior. The calendar can be used a substitute, but it requires that a new target (the day following this one) be reacquired upon every click, and thus is suboptimal.
This entry goes in tandem with a slight redesign that aims to provide more context, especially for individual entry pages. I had previously used a near-default Movable Type template, which limited itself to previous/home/next links. In the new version, this is replaced by a Jeremy-like sidebar that gives a bit more information. I'm obviously not the first to think about such things, and I don't claim my solution as being optimal, but I now have something that can be iterated upon.