Do not use invisible text. This is a modification of keyword stuffing. What the black hatter does is use the same color for the text and the background for any words they need to hide. Avoid doorway pages. In this sense we are talking about fake pages that only exist for the needs of attracting search engine crawlers. Again, you’ll see a short lived increase in your rankings, but might be dropped from the lists absolutely when the search sites check. Read and follow terms of service (TOS). Almost any site you use to help boost your rankings will have a TOS. When you start to look for loopholes or reasons, you are moving into black hat SEO territory. Therefore if that happens, take that as a sign to do the proper thing.
Doing the above things will go a long way towards avoiding black hat SEO strategies. This is vital as it will help you to naturally rank higher.