Here's how to make images for dark apparel look cracked.
1. Take a picture of some crackling/weathered paint that has a pattern you can use.
2. Resize it, if necessary, to 3000 pixels or whatever size you generally work in for shirts.
3. Greyscale it and do a threshold operation to convert it to black and white, adjusting the threshold to make the cracks and gaps black and the rest white. (In PSP7 it's under Colors > Adjust > Threshold)
4. Save the image as a PNG or other lossless format.
Now you have a crackle pattern to use as a mask. Here's how to use it.
1. Merge everything you want cracked in your design into one layer. (NOT a background layer or you'll lose your transparency.) If you have a brand, copyright, or URL you may want to not merge that so it will remain intact and legible.
2. Open the crackle pattern file.
3. Use the Canvas Size function (NOT resize) to make the size of the crackle pattern match the size of your design image. This will chop off any extra portions of the pattern so there will be no shrinking or stretching to introduce partial transparency. VERY IMPORTANT for dark shirts.
4. switch back to your design. Select the merged layer.
5. Apply the crackle image as a mask for the target layer. In PSP7 it would be Masks > New > From Image, then select the crackle pattern image from the drop-down, pick Source Luminance, make sure the Invert box is not checked, and click ok.
Now you have a crackled image.
Wear patterns on light clothing is similar, but with maybe a different source image and instead of a threshold operation you would play with the histogram to get the light and dark areas the way you want them. That will give you partially transparent areas when applied as a mask, simulating varied fading and wear.