Nothing brings a room down more than wimpy window treatments. If you can't do them right, and I mean custom, leave your windows bare. Bare can be beautiful, especially if the view is right.
If you need to have privacy then this is where you put your extra bucks.
Be generous with your curtains. You'll find that readymade panels are too narrow, sometimes 48" to 52". A sin! At least 70" wide per panel. AT LEAST. (Actual dimensions dependent on the window width and height.)
Unless they are sheers, they must be lined. Off-the-shelf options are usually not lined and when hung they disappear against the wall or window. Why bother? We often line them with off white, but you need to consider how that works with the exterior. For certain projects a color or even another pattern might be appropriate on the backside, but interlining will need to be used.
Don't let them puddle. There was a time when that was in, but not in years. We call it "breaking" at the floor. Also, never cut them off at the bottom of a window unless there is something built in below the window.
Proper hemming, on the sides and bottom, will make them hang right. No rippling at the sides and enough weight at the bottom to help them hang in lovely pleats. Only in the most casual of circumstance, like a tent or a tree house (yep, we've decorated those) have we not used some type of pleat at the top of a drapery panel. We often even add pleats to our shower curtains for a finished look.
Now, here's our biggest drapery treatment pet peeve: hang your drapery rods high, at least 10" above the window frame, if you have the room. In most cases, hang as high as the architectural features in the space allow. The added benefit will be the height it brings to the ceiling. If you have low ceilings, drapery panels are the first thing we will add height to the design.