The first thing to remember when purchasing interfacing is to buy a type that will be compatible with your garment fabric.  A rule of thumb is that the interfacing should be a lighter color and a lighter weight than the fabric it is going to support.  Color is generally never a problem, because most material sold specifically for interfacing is white.

For very lightweight fabrics you can use batiste or organza as interfacing, rather than a fabric made specifically for interfacing.  These sheer fabrics will give the finished garment a soft effect and must be sewn-in.  Organdy can be used if you want a crisp effect and other lightweight sew-in or fusible interfacing may suitable, depending on your fabric type.  Remember very delicate fabrics will not stand the heat of fusing; sew-in interfacing should always be used for these fabrics.

Make sure you match the care types for your interfacing and fabric. A dry-clean only fabric requires an interfacing that will stand-up to dry-cleaning.  A washable fabric requires a washable interfacing.

Medium weight fabrics like poplin, denim, linen, flannel, gabardine, satin, chino, velour, double knit, and stretch terrycloth require a medium weight interfacing for a soft effect and lightweight canvas interfacing for a crisp effect.  Poplin, linen, gabardine, and linen can stand the heat required for fusible interfacing.  On velour, knit, satin, and terrycloth use sew-in interfacing.

Medium weight fusible or sew-in interfacing is a good choice for heavy weight fabrics like heavier gabardine, corduroy, tweed, and canvas if you want a soft effect.  For a crisp effect, chose medium or heavy canvas interfacing.

Some sewers preshrink their interfacing before using it by dunking in hot water and letting it air dry.  I have never done this with fusible interfacing and I don’t think it is really necessary.  However, if you are using a canvas interfacing or some fabric not specifically designed for interfacing it may be advisable to preshrink it.