Use your phone to take a photo of your bike location, being sure to get some easy to find landmarks in the photo as well.
E.g. if you had just taken the photo above and your bike is the front centre one, those two trees should be fairly easy to find again when you come back.
If it still takes a while to find (or the search area is very big) and your phone has GPS, you can probably tag the photo with the location as well.
bicycles.stackexchange.com/users/96/Wilka
Aside from the fact that your bike is the only one you'll be able to unlock, the best thing for you to do is park in the same location every day.
That can be an invitation to have your bike stolen, however.
You can either: Make your bike easily distinguishable from the crowd.
Putting something brightly-colored on the bars might help, attaching a safety flag to the rear rack would be even better.
Either of these calls attention to your bike, again this is something inviting bike theft.
You could jot down a note to yourself for later -- "bike 3 arches down from the big ugly tree", for example.
What I would do is find three or four medium-traffic parking locations and rotate among them .
That way I'd only have a few places to search, but I wouldn't be using the same spot over and over.
bicycles.stackexchange.com/users/8/Goodbye Stack Exchange
In these kind of situations I use the tips that a guru taught me when I was learning the ways of triathlon.
In the transition area, where you leave your bike to collect at the end of the swim, you are quite often assigned a spot for your number (otherwise everyone clamours for the ends of rows).
So it's about landmarks,
"3rd row, lake-side, half way down, past the second tree", and memorable things, so the the flag (or strategically placed towel) is a good idea too.
Don't make the mistake of using temporary, movable landmarks.
That nice Cervelo might well have come out of the water (or work) before you :)
The key takeawayss here are to make them instantly and accurately memorable.
If you're in the same area day after day you don't want to to get too confused about whether you're remembering yesterday's or today's position, so @neilfein's rotation of possible locations is a good feint.
In triathlon you're trying to keep it simple (because your mind is inherently fuzzy when you're out of the water and trying to peel off a wetsuit), and at the end of a long day you probably haven't given much thought to your bike since about 2 minutes after you left it.
My bike is in a secure area in an underground car park, so I leave my helmet on the saddle which is generally still there, un-knocked-off, at the end of the day.
bicycles.stackexchange.com/users/425/Unsliced
Buy an electronic key finder.
Attach the keyring part to your bike, when you want to find your bike you press the button on the remote and the keyring beeps and flashes.
Range is 40 meters which should easily be enough.
bicycles.stackexchange.com/users/1509/Tom77
Decorate your bike so that it stands out a bit.
I usually paint the rear mudguard a bright colour and attach something colourfull to my handlebars.
And I try to park my bicycle in the same row each day.
bicycles.stackexchange.com/users/424/jilles de wit
I painted my mudguards bright red to aid visibility, but it doesn't actually help that much.
The thing that worked best for me was attaching a small pink teddy to the back of the pannier rack (like truckers do to their radiators).
It's eye-catching even in a crowd.
It's less good at Cambridge station, which is an extreme case, but works well in a long row of racks because it sticks out the back where it's visible as you approach the row.
I only don't do that any more because I bought a rear light that mounts there.
If you're coming back to your bike after dark, you're better off with unusually placed retroreflectives.
Most car accessory shops sell rolls of retro-reflective tape in various colours, and you can get sheets of stickers which you can cut to shape.
I always put a white triangle on my steerer tube where it's visible from the front, and a red stripe down my rear mudguard.
Since you're probably carrying your bike lights anyway, just turn on the front one and play the beam along the row.
Your own bike will wink back at you.
bicycles.stackexchange.com/users/3512/Dan Hulme
I have more than once had to spend time looking for my bike at the station, what’s your method to remember the rack you put it in?  Do any town have a system of numbering or labelling the bike racks so it is easy to find your bike?
How do you remember which rack you have left your bike in?
bicycles.stackexchange.com
<rack><parking><parking-racks>
bicycles.stackexchange.com/questions/1552
bicycles.stackexchange.com/users/132/Ian
A number od ways are recommended for finding your bike in a bike rack. Use your phone to take a photo of your bike location, being sure to get some easy to find landmarks in the photo as well.  You could invest in an electronic key finder. Another idea is to attach something noticeable to the back of the bike or the handlebars or decorate the bike in a noticeable way.  Perhaps you could park in the same location every day. Finally, you could write a note to yourself describing the location.  A good idea would be to note your parking spot with a picture, a GPS tag or a short description to yourself. You could also park at the same area, or alternate between a small number of locations to avoid theft. Alternatively, try giving your bike a distinct look to set it aside from the others. To this end, you could decorate it, paint it an usual colour, attach an object to it, or even place retroflectives around your parking spot. Finally, you might want to buy an electronic key finder.
A good idea would be to note your parking spot with a picture, a GPS tag or a short description to yourself. You could park at the same area, or alternate between a small number of locations to avoid theft. Try giving your bike a distinct look to set it aside from the others. To this end, you could decorate it, paint it an usual colour, attach an object to it, or even place retroflectives around your parking spot. You might want to buy an electronic key finder.