Don't Pay for Something You Can't See
A lot of people get caught up in the 4C's of diamonds. Especially, clarity. You don't always need a VVS1-VS2 clarity stone. There are SI1 and SI2 clarity stones that are completely eye clean and look just as good as higher clarity stones. When looking for stones in the SI1-SI2 range, try looking for inclusions that are white and on the perimeter of the stone. They're almost always going to be invisible to the naked eye. However, the one stone you should always try to get a VS2 clarity or better in is an Emerald. Emerald's are what we call the honest stone. What you see is what you get. They're usually not forgiving to inclusions because of the way they are cut.
It Doesn't Have to be in the DEF Color Range
A world renowned jewelry store that rhymes with Biffany's sells diamonds up to the J color range. GHIJ diamonds are near colorless and unless you put them right next to DEF diamonds, you'd never know that they aren't colorless. Another thing to think about is what type of setting your diamond will go in. Putting a DEF color stone in a yellow or rose gold setting is going to make the stone look warmer. If you're getting one of those setting colors, save money and get a GHIJ color stone. There's really no need to spend extra money if you don't have the budget to.
Don't just look at the carat weight, look at the stone measurements
The carat weight of your gemstone does not determine how big the stone will look. Focus on the millimeter measurements if you want a stone that looks big or covers your finger well. This is especially important when you're looking at stone shapes other than round. You can have a two-carat oval measuring 9.48 x 7.12 x 4.57 and another two-carat oval measuring 10.58 x 7.07 x 4.36. You'll get better finger coverage with the second oval.