Cleaning your collectibles is perhaps the most perilous aspect of collecting and there is a good reason why collectors are so apprehensive about it.
Abrasive chemicals and heavy handedness can easily wipe thousands from the value of your item, so if you don't feel comfortable with what you're doing, then consider consulting an expert. They may be pricey, but professional restorers and cleaners exist for a very valid reason.
However, there are a range of collectibles that can be cleaned easily at home and this is important in maintaining both value and eye-appeal. Prospective buyers will always be reluctant to purchase an item for its true worth if they have to undergo painstaking cleaning processes before they can enjoy it.
The first aspect of cleaning that we should touch upon is patina. This is the name given to the natural discolouration found on the surface of metals, which is highly prized as an attractive addition by collectors and actually adds value - do not remove this!
This is particularly evident in coin collecting. Take for example Heritage Auctions' recent sale of the Walter Freeman Collection of Three Cent Silvers, where the top lot was an 1855 Three Cent Silver which, despite not being the rarest offered, sold with excellent results due to its magnificent multicoloured patina.
The same applies for bronze sculptures or antiquities; a simple dust (possibly even a wax) will suffice for these. Do not attempt to scrub or use abrasive chemicals, as these will likely remove any patina built up over the years and leave you with nothing more valuable than a lump of metal.
However, with metals such as aluminium, steel and iron, you can use rust-removing chemicals to get rid of any unsightly patches. However, it is recommended that you start with warm water and a gentle soap before deciding whether chemicals are the best course of action.
Basic cleaning with warm water can also be used on porcelain, ceramics and glass. Despite their delicate appearances, these are actually some of the more hardy collectibles and can withstand a fair amount of scrubbing, but be careful - you don't want to remove any of the glaze or decoration in the process.
When it comes to art, especially oil paintings of any worth, you are almost certainly getting in over your head. Unless you have decent experience with cleaning antiques and art work, it is best left to the professionals, who use sophisticated technology such as lasers to clean tiny fragments without affecting the paint - an investment worth making.