Is it really just how well you write that makes or breaks your blog? That's part of it but it's also what you write and if it's appropriate for your audience and the goals you're trying to achieve.
Quality vs Quantity
Many people think more posts are better, but that's not always the case. If you write long meaty posts, you may actually fare better by not posting multiple times a day. This gives your readers time to digest your articles, and perhaps have a longer discussion about them.
In contrast, posting many times a day tends to be better for shorter posts. These could be links to other longer articles elsewhere online, or various other tidbits of info. If you go with quantity of posts, it pays to keep it up. People tend to be fine waiting a while for longer posts, since they take more time to write, but if your blog tends to have short posts as well as infrequent posting, it tends to make the blog feel rather dead and sparse.
Audience
Adding to the above, you're going to get different audiences depending on your posting style.
Link/News/Info Blogs
Blogs with many posts of lots of short info are very popular as well as informative. People tend to read these as "filter" blogs. Basically getting links and info filtered for them from various areas of the web. The Walker's Off Center blog is a good example of this, and is also our most popular blog.
The biggest tradeoff here however is that many of the posts on these types of blogs take you away from your own website to info elsewhere. While they convey a lot of info, most of that info is on other websites.
Article Blogs
You can offset a high post count blog with one that is more in depth. The traffic for these kinds of blogs tends to be less than link style blogs, but are also much more loyal. You also become the blog being linked *to* instead of the one linking elsewhere, because of the original content, which brings in new visitors. The Walker's Visual Arts blog is an example of this.
The downside here is of course the time involved in writing longer articles, and the general decrease in overall traffic.
Hybrid
Of course a hybrid is a combo of the two. Some short posts, some long. Most institution blogs start out like this. This can be a balancing act. Over time it may pay to split it out into two separate blogs, or to try to ride the fine line of the hybrid route. It all depends on your goals.
Other Styles
Wunderkammer
Wunderkammer ("wonder chamber") is a 16th century term for cabinets of curiosities, repurposed as a term that Seattle museology grad student Lynn Bethke coined about the format of how we do some blogging at the Walker. This type of post style is a collection of links to things you or your visitors might be interested in, but are not necessarily worth devoting an entire post to. They work as a great way to fill in the gaps when you're between the meat and potatoes of what your visitors are probably coming to your site for, "long and clean" posts. WebWalker is an example of this.
Pro to Pro
Most blogs are "museum to public". There is also the idea of making a "museum to museum" or "pro to pro" type blog. This means content aimed at other institutions or professionals in the same field as opposed to your partrons. You can create a completely separate community with this style of blog over the normal kind. The Walker New Media blog is an example of a Pro to Pro blog.