so i have this esp ltd b-50 (lefty) (http://www.worldmusicsupply.com/cart/pics.asp?Prod=B50-BKLH), and i've had it for a couple of years. i've started to notice that the strings keep going flat all of the time, and i'm wondering if it's the tuning machines. is this a common thing?
what would be a good way to solve this problem?

Do you always tune up to pitch? If you go sharp you have to drop below the note and tune it back up or you will go flat. Just guessing....

Does it go flat after it is tuned while playing? If so check to see if tuner screws need to be tightened.

when was the last string change?

i replaced the strings a week or so ago, and i stretched them a bit once i got them on. maybe it takes more time.
normally i tune sharp, and then flatten it out to find the pitch (i can hear the differences better this way).

Most basses tend to do this. It happens when strings are stretched, bent, etc. The only bass i havent noticed this happen on is one with a Steinberger headless tuning system, that stayed in tune forever.

i replaced the strings a week or so ago, and i stretched them a bit once i got them on. maybe it takes more time.
normally i tune sharp, and then flatten it out to find the pitch (i can hear the differences better this way).
You should really try to learn to bring them up to pitch. Because of slop in the tuning mechanisms, you are always going to end up going flat.

Most basses tend to do this. It happens when strings are stretched, bent, etc. I keep my basses in the basement. In the summer I can leave a bass for a week or more and they will still be in tune. And nobody has accussed me of having a light touch. In fact, now that the weather is more changable, I have to get used to tuning up before playing.

check strings. check the tuning pegs. i've had the screw on the underside (mechanical part) come loose, tighten that up if needbe (only if its an open style tuner)

Id hate to have tuning problems i have 2 ibanez's and theyre always pitch perfect. Did you make sure that the strings were wound at least 2/3 time round the machine head?

The tuning down to pitch rather than up is a very likely culprit here. When your last tuning adjustment is loosening the pegs, they will naturally loosen more or slip, causing you to go flat. When you tune up, the last movement of the pegs is tightening, and your strings will be less likely to slip loose.

I'm surprised more folk didn't jump on that, & that you haven't had this problem before. Always tune flat & come back up=no slippage. New string stretch probably factors in, too.

I've had this happen. Mostly the tension in the tuning peg isn't very tight, try this:

1. Lower the tuning of the offending string all the way to where it flops loose.

2. Pull on the string away from the tuner to tighten the tension on the post winds.

3. Gradually tune up.

4. Gradually let go of the string when ithe tension becomes too tight for you to hold it.

5. Follow the suggestions of the above, tuning up from a lower pitch than tuning down from a higher pitch.

i replaced the strings a week or so ago, and i stretched them a bit once i got them on. maybe it takes more time.
normally i tune sharp, and then flatten it out to find the pitch (i can hear the differences better this way).

Seanm said this but I've got to emphasize it - ALWAYS tune UP to pitch. Even if you have to retrain your ear to hear it correctly. The reason is simply the way tuners are made. There is always a certain amount of slop (also called "lash") between the spur and worm gears. That small space can mean the difference between in tune and out. When you tune UP, the worm gear is turning into the spur gear and pushing against the string tension. This keeps the string post (and it's spur gear) socked up tight against the worm gear and it can't move. Tuning down allows the small slop space to remain between the two gears and usually always leads to and out of tune condition. Next time, do both and see the difference this way - after tuning down see if your tuner key can wobble back and forth within the space of the gear lash. Do it up and try the same thing. I think this will show you the phenomenon pretty well.