Mistake: Strife

The New Testament teaches that strife can hinder your prayers, explicitly when it involves not being nice to your spouse:

1 Peter 3:7:
Likewise, ye husbands, dwell with them [wives] according to knowledge, giving honour unto the wife, as unto the weaker vessel, and as being heirs together of the grace of life; that your prayers be not hindered.

We have a New Testament warning that strife will lead to every evil work.  I would definitely consider sickness to be a subset of every evil work.

James 3:16:
For where envying and strife is, there is confusion and every evil work.

You must strive to stay out of strife!  Okay, I’ll rephrase that: Do everything you can to avoid being in strife with anyone!  It will stop your prayers for healing (or anything else for that matter) from working.