Don't use the same words all the time, try and vary your vocabulary as much as possible - the marker will find reading the same adjectives over and over extremely dull.
Don't use too many vague describing words such as interesting, beautiful or nice.
Never use describing words on their own - always try your best to include additional information to support them.
Slang or jargon words are definitely not the way to go about writing in an english exam unless you are told to write informal. If the examiner can't understand what you're trying to say, you won't get the marks.
This can be done by including metaphors and similes within your piece such as:
- He ran at the speed of light.
- Their eyes twinkled like the night's sky.
Try to pick out a few words before you go into the exam so they're fresh in your mind. The way to impress a marker is by using technical terminology and varying your vocabulary. Not to say you should try to change every word in order for it to seem more formal, but the odd word here and there makes all the difference.
3 words commonly used that can be replaced
Interesting - Fascinating, intriguing, gripping.
Nice - Pleasant.
Boring - Irksome, tiresome, tedious.