Bill Gardner / Getting a GTA Job
Guelph is a union shop, and GTA positions are handed out strictly in accordance with CUPE rules. The first point is that PhD students have guaranteed GTAs, if they apply for them, so all MSc students come after PhD students in the pecking order. Within any category, seniority points count. This especially helps students who were UTAs at Guelph before becoming grad students. Finally, students who have exhausted their usual program time are last to be selected and will rarely get a job.
Once your name is considered in order on the priority list, they look at your preferences. The worst mistake you can make is to only apply for a few courses. If these are popular and have already been assigned to higher-priority applicants, then you get nothing, even if there are lower-priority applicants below you. Your qualifications will be considered, but it is important to understand that "I TA'd that course successfully before" or "Dr. Foo specifically asked for/recommended me" have no bearing at all in the outcome.
Therefore the correct strategy is: apply for many courses, not just 3 or 4, and intentionally apply for ones considered as "unpopular." That way, you will likely get a job by virtue of your position on the list, even if it's not the one you preferred.