My only bit of advice I can give to anyone considering standing in the slips is to get yourself to 1st slip because in my experience, the catches are so much easier to take.
I appreciate it's not always the case but I have found that edges to 1st slip come from defensive shots or shots with a straighter bat. Also you can pick up the line of the ball from the bowler so much easier at 1st slip so you often know the the ball is heading in your direction before the batsman has even edged it. In contrast, whenever I have been stood further around at 3rd or 4th slip, the ball only seems to head in my direction when the batsman has flashed hard outside off and it arrives at pace + often dipping so these chances are so much more difficult to catch.
One other thing I would do is talk the wicketkeeper before the innings starts and discuss what the pair of you will do when the ball goes straight up. Couple of seasons I had this exact scenario when a spinner was bowling, the ball went straight up so I simply backed away to let the wicketkeeper takeover but he just stood there, it landed less than 3 yards from him and he then had the audacity to blame me for not catching it. In fairness that might speak volumes of the level of cricket I now play.