The best way to deal with pressure is give work (and anything else) as much attention as it needs. If it's to do something small at work, don't bother perfecting it, it's not worth the effort. That way, when it comes to exams/important jobs, you can give them a better go without too much stress.
Stress builds with time, so reduce that time by understanding that perfectionism can be a bad thing.
One of the reasons that so many try to perfect everything is because they see it as necessary, it's not! Even if you fail your exams, it's not the end of the world, and it certainly isn't if you don't work really hard a small job.