Independent students, over the age of 18 must live and have employment in Texas for at least 12 months without enrolling in school. According to the College Board, "Nonresident students who seek reclassification as residents must withdraw from school and be gainfully employed in Texas for 12 months" before they can re-enter an educational institution. You cannot enroll in school during any part of the time period used for establishing Texas residency. If you get married to a Texas resident, you must still follow the 12-month rule to establish residency.
The parents of dependent students or minors must reside in Texas for 12 consecutive months. For this purpose, the state assumes the minor is a resident of the state of the custodial parent. The College Board notes, however, that a legally emancipated minor "may establish his/her own claim to residency following the rules applicable to independent individuals 18 years of age or older." In such a case, the state treats the minor as an independent student.
The state considers students of the military residents of the state in which they entered the military. Exceptions to this, according to the College Board are "military personnel who are assigned to permanent duty in Texas and are not in Texas only for training." In this case, the state allows you and your legal family to pay the resident tuition fee.
If you hold certain scholarships, the state charges the resident tuition. According to the College Board, "Nonresident and foreign students holding competitive academic scholarships of at least $1,000 for the academic year or summer for which they are enrolled," must have a scholarship approved by The Texas Higher Education Board.