Dear Professor Snape,
Regards. I needed your help with something. My mother is skilled at Legilimency and she is one person from whom I cannot hide anything. All my Occlumency fails at her stare and she gets to know easily if something is wrong. Can you please offer me some advice?
Shielding ones thoughts from someone so trained is a difficult task. This is made all the more difficult as you are attempting to conceal secrets from your own mother. However, it is not impossible. You must focus on, not focusing. Simply put, when your gaze meets hers your mind races to conceal…in that concealment you have betrayed yourself. You must not think of your secrets.
When your eyes meet hers you meet her stare with casual confidence. You look into her eyes not in defiance, but in a calm and casual way that suggests you not only have nothing to hide, but are in fact not even attempting to hide anything.
Consistency is paramount. The more routine your interactions, the less likely she’ll be to suspect. Change indicates uncertainty, that leads to suspicion.
One final bit of advice… often times people secretly want to be found out of their secrets… mind that your heart doesn’t betray you.