Memorabilia has always been desirable, however, in the past two decades, awareness of memorabilia has led to more items coming out of the woodwork. From 'barn finds' and items stashed in the attic through to managers and agents selling off collected items, the memorabilia market has flooded with items, some of fantastic quality.
For example, we have a beautiful bill board image of Pete Townsend from 'The WHO', which albeit a fantastic image, is in dire need of restoration - conversely, we also have a beautiful guitar, signed by all five members of The Eagles which is in pristine condition. The one common factor which seems to be on the tip of everyone's tongues when it comes to memorabilia is the condition and hence quality of the item in question.
When looking for memorabilia, it is important to keep an eye on what we call potential value - This is how much potential the item has to sell, not to the general market, but to individual collectors. A signed photograph inscribed 'To Nick', for example, is relatively worthless to anyone except someone called 'Nick', but don't let that put you off. The photo in question is still signed and if you can find 'Nick', who happens to have an interest in the photo's subject, you can surely find a collector, and it will probably be valued higher than if it were just a photo with the signature alone.
Even without signatures, memorabilia of events and celebrities can be astonishingly valuable. The general rule being the greater the infamy of the subject, the more value it tends to add to the item. However, beware of items flooding the market due to popularity. For example, the Beatles are so popular and there is so much memorabilia out there that only the very rare and pristine items are considered really valuable.
Everything from concert posters to signed photographs to footballs to racing helmets are considered valuable and usually the rarity of the item only increases its desirability. Nowadays, event organisers tend to provide opportunities to collect memorabilia, more often than not oversaturating the market for that event and hence devaluing any collectible items. The best items always have and always will be collected by the fanatic, for the fanatic, and will invariably have been signed or collected as a one off. Much harder to come by, these items are incredibly desirable and the effort taken to collect them will always be rewarded with a higher value.