The only way to influence those that can be influenced is to be right and to be able to prove it without question. As the old saying goes, 1 test is worth more than a 1,000 "expert" opinions.

The only way to influence those that can't or won't be influenced is to give them the opportunity to fail and be patient. Notice I did NOT say "set them up for failure". That could hurt the company and your reputation. If what they're doing is truly a bad idea, they'll fail on their own. Be prepared to pick up the pieces. I've actually worked jobs in parallel so that when someone fails, I have fully tested code ready to rock at the drop of a hat. I don't do that to make myself look good, either. I do it as a concerned employee that earns his pay and has the welfare of the company at heart. Sometimes it takes more work than it should but it does have its future rewards. The manner in which you tell them "I told ya" will weigh heavily the next time you try to influence someone. The best method is to not say anything like that. If they don't already know, then you telling them isn't going to help.