Carrying luggage falls into two basic categories, hard luggage ie panniers and top boxes which are fitted to the bike, or soft luggage attached temporarily to the machine.
Nowadays there is plenty of choice available of both types and your choice will probably be made by the type of machine you are using and budget.
The advantages of hard luggage is of course it tends to be more waterproof and secure and can often be quickly detached and carried into an hotel like a suitcase. You can also get insert pannier bags which are often easier and a cleaner option to take in rather than detaching the whole box. We use these on our Pan Europeans, inside the fixed panniers. Down side is the high cost of fitting a set of hard luggage if your bike does not come equipped. After-market panniers etc can be eye-wateringly expensive. You can spend north of £1,000 on a set of hard luggage, but unless you are going to tour extensively several times a year then is really no need.
Soft luggage like throw over saddle bags are far more flexible and of course cheaper, but will need to have any items carried in waterproof ‘drybags’ inside in case of leaks, even those nifty bags with waterproof covers are not 100% reliable in a downpour. Light weight drybags designed for camping are very cheap on eBay and you can easily pack everything into one before loading the drybag into the saddlebag/ pannier/ topbox. It is very important when strapping this type of soft luggage onto the bike, either by its own straps and or ‘bungees’ that it is secure and will not move. The very last thing you want is something falling into the back wheel and locking it up, throwing you from the machine in the process !
We recommend using ziplock plastic bags for very sensitive items eg electronics, documents etc. We use freezer bags from IKEA which are available in A4 size with a double “zip” and are very hard wearing, opening and closing a dozen times with no issues.
Always read any manufacturer’s instructions on loading and try and put weight as low down as possible so as not to affect the centre of gravity and thus the machines handling. A heavy top box really does have a huge negative effect on how the bike handles. Use netting, sticky plastic film or non-slip carpet material, to avoid damaging any paintwork or seats when using soft luggage.
A small magnetic or strap-on tank bag or small rucksack is always very useful for storing your original documentation and important valuable items. This can then be easily removed from the bike and carried with you when you leave it parked (for coffee or lunch or whatever) for greater security.
Avoid carrying your luggage in a rucksack on your back on the bike as it is tiring and leads to rider fatigue. You also have to take it with you at all times, even when having a quick coffee!
Lastly the golden rule here is to do a trial pack, loading and unloading technique is much easier to refine in the garage than on the side of the road in the dark after a long ride. Then having packed and loaded your machine, whether it be hard or soft luggage, undertake a reasonable length journey to make sure things are tight and the load has not upset the bikes balance before you finally set off. Check your tyre pressures and suspension adjustment, and you are all set!