Firstly, why do we need to get a lower rating?

1. Some players need to play lower contests, but their ratings have exceeded the limit
2. To compete the last page of rating list and become a negative LGM

Mostly, you have to submit at least once to change your rating, whether it's an ACM contest or a CF contest.
+ If it's an ACM contest, you have to reach a rank that low enough. If your rating is so low that it's increasing even you just solve one problem, then you should finish it in a score of 0.
+ Or it's an CF contest, you could finish the contest without any AC. But if you want to get a negative rating, you have to do more! At least you have to get a negative score to get a low rank.

Watch the rank list. There may be someone else competing with you to get lower score, so the best way is to AC 1 problem and Hack Failed for many times. If you are sure you can make a code that could Failed System Test at that problem, then do it. If you want to do, you can use some trick to Hack Failed as quickly as possible, but if you are Skipped or reported because that, you would failed this way.
There are always some strange ways to get a really low score, so if you find a player that get a much lower score than you and you couldn't get score as low as him, you have to report him/her to an administrator or try to get Skipped of this contest so you wouldn't get an raising rating.

It takes a long time to get a low rating. If the contest is Semi-Rated or Unrated because some reason, don't be sad. All you have to do is focus on next rating-falling contest. Also, you shouldn't use two account, one play as normal and other getting lower rating. The second one will seriously affect the first one. Check the last page of rating rank list, and learn from those with negative rating.

Hope you could have fun in losing rating :P