Guest Posting - Good Practices and Questions

I am planning to start Guest Posting and would like to know some best practices.

1) People say google can track which is a guest post and will dilute the value of the link. So is it something like if we post on websites that offer guest posting we will get lesser value. (Am not talking about shit sites created for the purpose of author accounts and guest post, I mean some good site with DA30 with clean records)

2) What are your successes with guest posting. Have you tried guest a how does it work for you.

3) Also would like to know some best practices and things to avoid.