The 'typical' SGL is usually a former GL of several years' tenure with a good performance record (LOS, TM prod, BTS scores, etc). Most seem to come from other DCs around the country, so I guess a willingness to relocate would help. Some degree of asskissing prob doesn't hurt either.
Sorry I can't give more detailed info. I'm just a lowly TM. The workings of GMs, SGLs, PCs, HRMs and HRBPs are pretty much shrouded in mystery to us TMs.