Dragging Your Feet - One of the big mistakes you can make when you are making an offer is to drag your feet. "Time Kills Deals". Have your financing in place, know what you can afford and be ready to go so you can make an offer in a timely manner.
Purchasing at the Top of your Budget - this leaves no room to negotiate with the seller. Don't make an offer at the top of your budget if possible.
Trying to Lowball the Seller - This can cause the seller to not even want to negotiate with you if you do make another offer--especially in a seller's market with low inventory and high demand. Ask a sales professional before so you can make the right price offer.
Waving Inspections - It is always good to keep inspections as an option even if you don't do them in the end because you never know what may come up. It protects the buyer.
Present Yourself in the Best Light - If you are in a multiple offer situation, present your offer/yourself in the best light. Don't be nit picky about details, make sure you look as good as you can on paper with a clean offer.