First off I try my hardest to work with as much natural fiber content as I can. Linen, wool, cotton, rayon, silk on occasion. Even with varied weaves the way these fibers behave is relatively consistent.
A lot of it starts with fabric selection. When I'm out shopping for yardage, I try and not get too dazzled by what the fabric looks like unless I feel I can deal with it's content. It can be limiting: no polyester (if I can help it), no lycra or stretch. Lycra is basically a rubber band woven into the fiber of the material. The more that is exposed to water from washing and heat from a dryer, the more that stretch brakes down. Often you will get a rotting rubber smell in the fabric, or it can just start to tear and rip. Gross, right? Fabric with stretch often has a shorter life than non stretch fabrics. Thats one reason I tend to stay away.
When I have selected yardage, the first thing I do is look it over. Any spots, tears, etc. I often buy my fabric at discount places, and often their bolts are end bolts or rejects. Sometimes there can be something wrong with them.
When I decided to sew my garments I made a decision that most of them would be machine washable. The first thing I do when I get new yardage is wash it. I NEVER make a garment without pre washing the fabric. Washing removes the starch/excess dyes/shrinkage that can unexpectedly come with new fabric. I still can't believe people don't do this. There is no reason whatsoever to not wash your yardage. And if you have silk or some other delicate fabric that you might want to have dry cleaned, dry clean the yardage before you make the garment. Yes, you heard me.
If the fabric is a loose weave, or denim, or something that will unravel, I'll overlock the edges of the yardage that were cut. This can save inches of fabric and just relieve the worry of having stray threads wrap and tangle around other things in the wash.
Tips for washing fabrics.
Once the fabric is prepped with it's overlocked edges, now is the time to decide how to wash them. There are some of my general rules.
I wash all cottons the same. Darks together, lights together on hot. I will tumble dry these as well, unless the cotton is a lighter fabric, then I typically hang to dry. I always wash warm hang to dry my finished garments.
The more you wash linen, the more that crispness goes ways because the fiber will keep braking down in the wash. If you want linen to keep is crispness then you will want to have it dry cleaned. But for me, I wash it on warm at least once to cut the shrinkage then I'll have a finished garment dry cleaned. I always hang my linen yardage to dry.
Here is a fabric I sometimes hand wash. I just put a bucket in the kitchen and fill it with warm water, some soap (delicate soap if you are worried about bleeding) and hand wash it for a bit. I'll rinse and hang to dry. For me, with finished rayon garments, I always hand wash. If I have like 5 yards of rayon, I'll put it in the machine on warm, then hang to dry.
I think I am one of the only people I know who will machine wash wool. I have no problem with it. I actually like it when it shrinks (and most likely it will) a bit, so I actually buy about half a yard more than I need.
One time I acquired a lot of wool and some were crepes. I didn't know how the crepe would handle a machine washing so I made 5x5" squares, overlocked them and tossed them in a warm wash. The only one to shrink was the cream crepe.
Synthetics and synthetic blends
Often these handle the most predictable. You can machine wash and tumble dry pretty easily. I still tend to hang dry my lighter synthetics.
*Special handling fabrics*
Muslin is actually cotton but if I'm using it as interfacing I'll wash it twice to make double sure I have gotten rid of shrink. If I'm using it for draping I don't wash it at all.
With these fabrics you have to be more careful. Check over them throughly to make sure there is no rotten or worn patches, tears, etc. I always hand wash vintage fabrics. I consider their treatment a step below my delicates, but not by much. I use a luke warm warm water and try to wash each piece separately because you can never be sure in how the old dyes will react to a wash. I squeeze the fabric (never twist) and hang to dry.
If it's vintage cotton, I'm a bit more rough and sometimes I'll put it in the washing machine.
Notions and Trims
I try as hard as I can to get 100% cotton notions and trims. Rick Rack, twill tape, bias tape (make your own!) etc. I will wash these by hand in warm and hang to dry only because I don't want the added stress of a machine wash to stretch and wear them out. Zippers I tend not to launder prior to using.
Once the fabrics are washed I make sure I smooth them out as they dry so I minimize ironing. If the selvage is all wrinkled I often iron that edge prior to cutting the garment. If the whole yardage is a big wrinkle, I'll iron it all out first then cut. I think you all know what happens from then on out :)
I hope that helps everyone. Nothing too special, but just like tracing off a pattern, I consider pre washing my fabric as part of the garment making process.