Creating a strong pattern
The first thing to do is use eight or nine nodes in your pattern. A hacker or snoop trying for a quick score won't bother attempting patterns with that many nodes when most people only use four or five.
Even with that many nodes, however, if you spell out a common shape like a letter, or start it in a corner, you're improving the attacker's chance of getting in. You want a pattern that starts in the center or edges and crosses itself several times.
Here's the type of pattern Marte Løge suggests (obviously you shouldn't use this exact pattern, though):
It would be hard for anyone to guess this, and even someone looking over your shoulder might not catch it with a single look. Speaking of looking over shoulders, another security measure you'll want to take is turning off the "make pattern visible" option.