Bladders come in all shapes and forms, the most common capacities are two litres and three litres. If you are planning a long day out, go for the larger variation, after all you don't have to fill it all, but in hot weather, one litre per hour isn't out of the question.
When it comes to choosing an actual bladder, things get more complicated. Despite what manufacturers tell you, most bladders will end up tainted in taste terms if you regularly use sport drinks and most will, eventually, die after extended service.
In day-to-day use, a useful feature is some sort of anti-microbial treatment which will delay the time it takes for mould to grow in an unattended system. Different companies use different materials for the bladder - such as beverage-grade polyethylene, a PE film to minimise plasticy tastes or some even have a life-time guarantee on their bladders.
The opening method of the bladder is the other big consideration. Threaded caps can become sticky, especially if energy drink gets on the threads. A really big opening like a simple fold-over and clip version is easier to use and makes cleaning access to the bag simpler. Tip: A smear of vaseline can help keep sticky lid threads running smoothly.
The Bite-valve: With a bite valve, you're looking for two things - good flow rates and simplicity. Mechanical-type valves with springs and hard innards are all very well, but seem more prone to failure, contamination and simply don't flow as well, though most will be adequate. A lock-out facility means your hydration system is less likely to spill its innards in the back of your car, but you still need to remember to switch it on. Finally, a right-angled, ergonomic valve will put the mouthpiece at a more user-friendly angle.
The Pack: It is probably better to buy the pack for the job then add a decent hydration system to it rather than buying all in one, unless you are getting a very good deal and were only after a small-capacity biking or running pack - most purpose-built hydration packs aren't large enough for general use.
Generally look for a decent-sized hydration sleeve - especially if you have a three-litre bladder - with a hook or straps at the top to hold the top of the bladder. Check how easy it is to route the hydration tube out of the pack and whether you can use either side, or there's a central opening. Finally, the shoulder straps should have some retainers to stop the tube from flapping around.
Care And Maintenance: New users are often bemused by caring for their hydration packs, but in reality it's quite easy. Rinse straight after use and either store in a freezer or dry carefully and store open and dry.
If you do get mould growth, you have a number of options. You could invest in a set of specialist brushes that will enable you to clean the tube thoroughly, rinse afterwards with warm soapy water then dry out as before. In bad cases, use the sterilising fluid sold to rinse babies' bottles and soak overnight.
You can buy specific sterilising tablets or some users swear by denture cleaning tablets. If you have a bad attack of mould, dismantle and thoroughly clean the bite valve as well. The alternative is generally an upset tummy, which is the last thing you want on the hill.
Brrrrr.... It's Freezing: You can also buy neat little insulated tube and bite valve covers, but if it is really cold, they may only have a minimal effect. Blowing back fluid into the bladder after drinking helps to prevent the tube from freezing solid - it always goes first - and starting off with warm water in the first place will also help.
When you get well below zero however, things will tend to freeze - so be prepared - you don't want to get left with nothing to drink after a mammoth climb with hours of your route still to go .....