Strings can be a fun or frustrating part of the Zentangle process. In my classes it's not unusual to get the 'deer in the headlights look when I tell my students to create their own string. They are crucial to getting you started but they shouldn't totally control what you do with your piece. That's probably why there are so many challenges put out incorporating string ideas.
On Tanglepatterns.com there is a section called string ideas. Its a wonderful resource with 50 strings drawn out for you. I've been watching this site as new ones are added and even have one of mine featured. (with more to come I hope) The site had a special surprise for its 50th string idea. It's one drawn by Maria Thomas, the wonderfully gifted co-creator of Zentangle! It is so elegant in its lines that I had to try doing tangles with it. I'm not sure it is everything I want it to be - but remember, there are no mistakes in Zentangle, so I can always revisit this string later and see ho I'm inspired next time.