Before you start, consider what your goal is - are you a saver or are you trying to "invest" in the hopes of a big run up in price? If you're a saver, great. Stack away, dollar cost average, and you'll probabl think of your stack in terms of ounces, not "dollar value" either way. If you're looking to invest? Maybe not such a great thing, since silver and gold are MASSIVELY manipulated (and have been for a while) so you're not going to make any sort of big killing quickly.
1) If you don't HOLD it, you don't own it - buy PHYSICAL (aka "phyzz"), never PAPER.
2) Unless you're a hard core coin collector, stay away from numismatics ("collectible coins"). Yes, they may have some "collectible" value now, and even into the future, but you're buying for the value of the silver. Most collectible coins are vastly overpriced relative to their actual silver content, and when you're looking to buy food or fuel in a SHTF scenario, nobody is going to care that the quarter you have is "rare" and should be valued more than any other, to be honest.
3) Stick with things you know and most people will recognize (American Silver Eagles, pre-1964 "junk silver" coins, esp. dimes and quarters which are smaller quantity and easier to spend for smaller purchases). Nothing wrong with getting some reputable rounds or bars from reasonably well known sources either, but the more generic the item, the harder it may be to convince someone it's actuall silver sometime down the road.
4) Consider getting a cheap but decent pocket scale. There are several decent options for under $20 on Amazon and other sites, and they can be very helpful to figure out the actual coin weights for junk silver if you're trying to establish the value of what you're buying or selling and/or ensure that not TOO much has been lost to wear from regular use in circulation and all.
5) GO TO YOUR LOCAL COIN SHOP. If you're lucky, or live close enough to one, it's often a really GREAT way to make purchases and stay reasonably under the radar, and you can often get some good deals. Never hurts to be friendly to those guys, they can give you a heads up to anything they hear and may let you sort thru their junk silver bins to pick your coins as you go (if you develop a friendly relationship or they are not being jerks). Personally I'm very distrustful of making any sort of bullion purchases online and using CCards which can be reported on and traced. Cash 'n carry, keep the sales helping the local businesses, and keep it all nice and on the down low.
6) Don't talk about any stacking you do (details) with anyone - MAYBE a significant other, but that's it. The less folks know, the better off you are. Nuff said there... loose lips sink ships and all that. Never give anyone even a reason to be curious, ya know?
Most important - have fun. It's actually fun and exciting to do this, and a decent step in helping to be prepared and preserve wealth in a form that's not going to lose value due to QEInfinity or anyting else. Figure out what you can afford, and probably the smartest way to do it is to establish a rhythm for yourself of making regular purchases - "stack the smack", look for times when the prices are manipulated down to buy a bit more of course (if you can), but dollar cost averaging is your friend and right now, it's still very much "cheap" compared to where it's likely to end up when the system eventually breaks and the dollar goes bust.