Choose a template that suits your blog content. It not only adds to the blog's beauty but will also accentuate the content well. Bad design is always a put-off to readers.
Respond when someone likes or comments on your post. Visit their blog and send your comment love. Courtesy brings in more readers and friends.
Formatting - Your post need not be a circus. If you are not good with formatting, then stick to a theme that is light and descent in colors and the post layout. If you are good with trying new stuff then use that skill to better express through colors, fonts and layouts. Avoid colors that are dark and fonts that are overtly bright.