I re-proof my gear all the time, it;s easy. First is to make sure you clean the goods with soap made for high tech gear and the key is that the soap can not have any fabric softener. You can spend big $$$ for niki or other soaps or just use woollite in a powder form. I keep a box of woollite and use this for 80% of my gear cleaning but also use Niki for select cleans.
2nd you need to purchase some DWR...I use McNett Revive, it's one of the best. Spray it on the garment when wet and dry at a low heat. Some garments like wool can not be dried in a machine and need to be layed flat. In this case you can use a Niki DWR that can be room dried without the need of heat from a dryer for activation.
Heat alone can sometimes re-new some DWR's like McNett, so a run through the drier or a couple hours on the patio in direct sunlight can bring it back.
Check-up on the subject by doing a google search, as well the MFG of the fabric. Also when applying the DWR, lay it on heavy in key areas like shoulders, butt, seams, zippers and cuffs.
Wash-in DWR vs. spray-on is a whole other topic, but the rule of thumb is use spray on with membain-based cloth like gore tex or windstopper and use wash-in on softshells, although I use spray-on for everything and wear schoeller softshells 80% of the time.