Recognizing that fact and adjusting your writing style according to each medium is a must-do. Do some homework and research your audience to develop this understanding. The greater the restriction on word count, the greater the likelihood that it will take you longer to write. If you expect your audience to read through all your copy, you're going to be sorely disappointed. If you're a novice copywriter, you'll have more success if you get the important points across early on, which not only gives the reader a reason to stay engaged, but also better ensures your overall chances of communicating your message.
All good copywriters look to trim unwanted words and sentences from their writing.
If I can say in one sentence what someone else has had to take three to say, I'm doing a damn good job. There's nothing I hate more than disjointed copy. If the information you're trying to present has any timeline whatsoever, unless you have a specific purpose for jerking the reader back and forth through time, be sure to put your facts into chronological order. Confusion just leads to disgust which leads to abandonment of your message. The big man sat down at the table to eat dinner.