The LYV 'C.A.R.D.' principle.
How to value your records
Condition (The good, the bad & the ugly)
Condition matters. If you use the Record Collector 'Rare Record Price Guide' (as most dealers do in the UK) then you'll see that they have seven different levels from 'Mint' to 'Bad'. Advertisers will also add their own intermediate levels such as EX- or VG+, both of which probably mean that they would like to use the classification EX but the record is really only VG.
Obviously, you want to get the best condition available but don't put off buying until you find that super, pristine copy. It may not exist. Buy what you can find and then upgrade later if a better one surfaces. You can always sell the old one to somebody less fortunate.
Store your records properly so that they don't deteriorate and lose their value. Judicious cleaning may even improve their grading.
It should go without saying that you need to take greatest care when playing records. Use the best equipment that you can afford and keep it well maintained. A chipped stylus will be instant death to you most precious vinyl.
The LYV grading system is reasonably easy to apply and unambiguous.
Age (Years)
Vinyl has been around for over 60 years and is still being produced. There's nothing that you can do to alter the age of a record. It does that all by itself as time goes by.
Age will increase the value of a recording but is usually a minor factor. Just because a record was made in 1956 doesn't mean that it will command a higher price than one made years later. However, it will certainly be more difficult to find older records in above average condition.
Rarity (Quantity in circulation)
Rarity can have a huge effect on value. Chart 45s are unlikely to command high prices. The notable exceptions being first issue Beatles and the like and even then they are still quite affordable.
Private pressings and withdrawn or cancelled releases will have rarity value. Records that are known to exist in very small numbers, maybe only one or two copies will always have relatively high values.
There's nothing much that you can do to affect the rarity of a record except perhaps issuing your own 'limited edition' or buying up (and destroying?) all the other copies.
Be warned that rarity does attract counterfeits and some are very plausible. Unless you've actually seen a known genuine copy, and can make comparisons, you could easily be duped.
Desirability (Who loves you, Baby?)
Desirability can go up and down like a yo-yo. One day a record is 'in demand' and the next week you struggle to give it away. Events and fashion can have an enormous effect. When Michael Jackson died his records (temporarily?) increased in value. If punk or some other style makes a comeback then early recordings will suddenly be more sought after.
If you are a record dealer then this is where the risks are and where good profits can be made. It's like the Stock Market. You need to know when to buy and when to sell. Also, know your customers, and who will be interested in particular items. It's pointless offering Heavy Metal to a Reggae collector.