Don't string too many words together with a single hashtag.
If you tweet with a hashtag on a public account, anyone who does a search for that hashtag may find your tweet.
Don't #spam #with #hashtags. Don't over-tag a single tweet.
Use hashtags only on tweets relevant to the topic.

Do be specific when using hashtags
Try and hone in on a passionate community that shares an interest in one specific theme. The more specific you can get with your hashtag, the more targeted your audience will be—and a targeted audience generally means better engagement. If you don’t have your own business hashtag, find one or two existing ones that really fit the photo.
Say, for example, your business sells baby products. Instead of using #parents—resulting in parents of children of all ages—opt for #newmom. The hashtag #newmom is specific to mothers of newborns—your target customer.

Steps to use hashtags:
Don't string too many words together with a single hashtag.
If you tweet with a hashtag on a public account, anyone who does a search for that hashtag may find your tweet.
Don't #spam #with #hashtags. Don't over-tag a single tweet.
Use hashtags only on tweets relevant to the topic...