There are some things you should consider:
- prepare a guest posts guidelines- you can include things like: minimum number of words e.g. 1500, maximum number of links, topics
- topic of a guest post should be relevant to your existing content
- check posts for plagiarism- uniqueness matters
- check posts for quality- remember that you don't have to accept every post people send you. Everyone does it to get a link back to their website but some people forget that their posts should provide value to the readers
- edit post if needed, check for spelling etc. However, confirm changes with the author.
- if you exchange posts- check the website of other party- don't link to your website on poor quality sites
- don't accept too many guest posts- they should enrich your blog, not to replace your posts. Different approach to a topic can be interesting to your audience but you can lose them if they see only posts from random people.
Hope that helps!
