Let me clear up one thing. Georgey was technically correct but left out one very important detail.
Not only must you live in Texas for 12 months prior and have a job, YOU CANNOT BE ATTENDING SCHOOL FULL TIME DURING THIS TIME PERIOD.
I've been attending college in TX for 5 years, and I'm an independent student with a job. However, I have been attending full time schooling every year so that means I'm not considered a TX resident.
Now, it turns out that these requirements are not state law. They are simply guidelines set down by the THECB (Texas Higher Education Coordinating Board). The schools themselves are free to follow these guidelines or not. For admissions purposes, all of the schools follow them. However, for tuition purposes, several of these schools DO NOT follow the THECB guidelines.
For example, at UTMB and UTSW they will often give an out of stater who is admitted the same instate tuition as a TX resident, regardless of whether or not they have lived and worked in TX for 12 months prior to attending.