When you have two yards of a fabric that is just perfect for something that requires four yards, you haven't a problem: it's obvious that you need to buy more fabric, if it's still available. If you can't get more, you have to set the fabric aside for a smaller project, or select two or more pieces and do color blocking or patchwork.
When you want to make something to go with something already made and can't get more fabric, there's nothing for it but to select a harmonizing fabric and, perhaps, trim it with the scraps from the garment you want to match.
So what I intend to discuss in this article is the infuriating case where you have a piece of fabric that's big enough, you know it's big enough, but somehow, no matter how you lay out the pattern, it won't quite go.
Your first step should be to check the pattern for waste.
If it's your own pattern there won't be any, of course, but a new commercial pattern is bound to have such things as a seam allowance that's cut at 5/8" and then trimmed to 1/4"; trimming the pattern instead will make the sewing easier in addition to giving you a bit more wiggle room. On the other hand, if you are using this pattern for the first time —as is strongly implied if the waste is still in it— you may need some of that waste for fitting.
If the pattern is cut on the fold, ask yourself whether it's worth while to duplicate pattern pieces as required to lay out on a single layer of fabric. It's often possible to interlock pieces more economically on a single layer.
If it still won't go, very cautiously consider letting the pieces overlap a little. There are usually places where it won't hurt a thing to take a little notch out of a seam allowance or nip a corner off a seam allowance. This method can very easily lead to a disaster that ruins the entire piece, so think three times and cut once.
The next expedient is to piece the fabric. You can cut a pattern piece into two pieces that can fit in among the other pieces. (Don't forget to add seam allowance!) Or you can lay out all the pieces but one in a fashion that leaves two scraps that you can sew together before cutting out the remaining garment piece.
Other examples of piecing: Cut a wide hem down to a seam allowance, and sew on a facing. Change cut-on facings to sew-on facings.
Which suggests making parts that don't show from a different fabric; with a little thought, this can be made to appear as a design choice.
Pocket bags can be made from a thin and tough fabric faced with the fashion fabric where the inside might show. (But you're probably already doing this to make the pockets wear better.)
And then there's making parts that do show from a different fabric. If the pattern doesn't include small parts that can be contrast trim, you can piece larger parts — for example, add a band of contrast at the bottom of a sleeve, or make a piecing seam appear more deliberate by inserting a contrast band.
Patching this way can lead to a hobo hodgepodge, but with thought, you can end up with a product more elegant than what you had in mind at the beginning.
On a related thought, you can make one top form an outfit with two different bottoms by making the large pieces match one and the trim match the other.
I read —perhaps in one of Elizabeth Zimmermann's books— of a sweater that was made to match several outfits by working a stranded pattern with one yarn to match each outfit, plus a neutral background color. But in sewing, I think that more than two would be pushing it.
Once upon a time, I had a remnant that was too short to make a pair of slacks, but plenty wide -- so I turned the pattern at right angles, using the cross grain for the long grain and the long grain for the cross grain. It worked out beautifully -- except that the pants were too tight. The cross grain has more give than the long grain.
You can eliminate a hem entirely and substitute a binding. If you can match the color, a floor-level binding won't be strikingly different from a hem. Binding is particularly appropriate for edges that get heavy wear; when the binding wears through, you can pick it off and apply a new one.
You can save a surprising amount of fabric by switching from long sleeves to short ones. But make sure the fabric is one that you can tolerate in short-sleeve weather. This is really a subset of "set the fabric aside for something smaller".