Don't use invisible text. This is a modification of keyword stuffing. What the black hatter does is use the exact same colour for both 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 tempting search website crawlers. Again, you will see a short lived increase in your rankings, but may be dropped from the lists totally when the search engines find out .
Read and follow provisions of service (TOS). Almost any site you use to help boost your rankings will have a TOS. After you start looking for loopholes or excuses, you are moving into black hat SEO territory. Therefore if that occurs, take that as a sign to do the right thing.
Doing the above things will go a long way towards avoiding black hat SEO techniques. This is crucial as it will help you to naturally rank higher.