With the myriad of products out there to care for your gear, it can be hard to decide what is the actual best product for your needs. This issue requires specific explanations for different kinds of gear, so I've broken it down into the following three categories:
Footwear: Footwear comes with all different materials and linings. In general, if you have a decent hiking shoe, it will have some form of waterproofing, whether this be Goretex or an in-house brand such as Keen Dry. These linings keep your feet dry, but they allow the outer material to saturate first, meaning the shoe will not dry quickly. One way to help with this is to use a footwear proofer that can come in sprays or rub-ins. Sprays are most useful on footwear with a great deal of mesh or fabric materials. Rub-ins are the best for leather. If you have a solid leather boot, whether it has a waterproof lining or not, treating the leather is vital to ensure you get the full life out of your boots. Leather that becomes saturated and then dried will become dried out and brittle leading to eventual cracking. The way to stop this is with a paste wax, which both waterproofs the leather and moisturizes it. Whether using a spray or rub-in, it is most important to ensure your footwear is clean, as any dirt will mean the treatment won't adhere to the boot well and will wash off after a couple uses.
Technical outwear (shells): All waterproof/breathable shells come with a DWR (Durable water resistant) coating. This coating causes water to bead on the garment and roll off. In order for this coating to work effectively and for your garment to retain its breathability, you have to keep your jacket clean. However, many detergents are harsh and actually strip the coating off. Because of this, it is recommended to use a specific "Tech Wash" which will clean your garment without striping off the DWR. These come as a washer detergent or spray cleaner. Afterwards the coating can usually be resotred just by throwing it in the dryer. However over time you will find that your jacket just doesn't bead water like it used to, no matter how much you wash it. Instead of buying a whole new jacket, the solution is a simple reproofing treatment. Reproofing products come in three varieties, Spray and hang, wash in and dry (the dryer is essential to seal in the DWR treatment), or a combo wash and treat (a tech wash and reproof in one). Once these have been applied, your jacket should perform like new again.
Down Garments: Down is such a finicky material that I think it requires its own category. Down has amazing insulating abilities, however as you probably know if down gets wet it becomes virtually useless. What you may not know is that our body oils over time can seep into our down garments and reduce the down's insulating ability. The solution to this, of course, is to wash your down garment, however regular detergents can actually leave a residue on the down fibers reducing their efficiency. The way to fix this is to use a specific down cleaner, which gently cleans without leaving a residue. It is also very important to dry your down garment thoroughly after washing it to prevent mildew build up. Try to avoid air drying as this still allows mildew to build up inside (and will take days), but rather throw it through the dryer (on gentle cycle) for as many cycle as it takes to become completely dry (probably at least 3 times) This will give the down maximum loft and ensure that no mildews builds up.