I've been using Lightroom since version one (wow, that's 7 years ago!) and while the basics of the software are pretty self-explanatory, I thought I'd share a few little tips, tricks, and features that make my photo life easier, better, and less aggravating.
When you import RAW use 'Copy as DNG'
To begin with, always import your RAW files as DNG. DNG files have a number of advantages over your camera company's native RAW file. They keep all the metadata internal, so no more of those pesky little .xmp files to keep track of and copy around. The file format is open and based on the TIFF spec so you'll know that you'll be able to open those files up for years to come. DNGs use lossless compression of the data so they take up about 20% less space on your hard drive, and it's the recommended file format of ASMP and the Library of Congress ('nuff said).
So when you're importing your RAW files, have Lightroom convert them to DNG in the process. It's a bit slower because of the conversion overhead, but then you never have to think about it again.
Go into your catalog settings and check "Automatically write changes into XMP"
Out of the box, Lightroom keeps any changes you make to your RAW files in the Lightroom catalog file. That includes anything you do in Develop or Library which means exposure, white balance, ratings, starring, keywords, etc. Otherwise known as pretty much everything you to in Lightroom. Having all of this in one corruptible file makes me nervous, so I go into my Catalog Settings 'Metadata' tab and check "Automatically write changes into XMP". Even though it says XMP, this also writes the metadata into DNG files.
The practical upshot is that if your Lightroom catalog gets hosed for any reason, almost everything about your files is safe in their individual files. Just create a new catalog, drag all of your photo folders in, and you're back up to speed without loosing a beat. It's also handy if you send a DNG file to someone else: all of your changes will go with it. Adobe used to warn that if can slow things down a bit to be doing that many writes all the time, but I haven't noticed. Consider it an insurance policy.
Set Default Develop Settings
Do you find yourself always making the same changes to all your images just to get them to your starting place? Maybe you always like your shadows opened up a little bit to start, or a little more contrast, a touch of small radius sharpening, or perhaps you like things a tad less saturated. Well you can set your own personal default starting place.
Just start in Develop with a fresh file, make the changes you want to start all future imported images with. For example, I make sure that 'Enable Profile Corrections' is selected under 'Lens Corrections'. Next, hold down the Option key on a Mac (Alt I think on Windows), and the Reset button at the bottom of the right sidebar will magically become 'Set Default...' Click the button and from now on, that's your new starting point.
Search By Metadata
Sometimes it's fun to mine your catalog for some insight into how you shoot. If you press CMD-F while in the Library module (BTW, the fast way to get back there is to press the G key on your keyboard. Jumps you right into the Library module in Grid view) you'll open up the top bar that'll let you search through whatever folder you're in by all kinds of criteria. For example, you could filter by all your images that use a specific camera, lens, or aperture/shutter speed setting. I've done this myself and realized that I tend to like photographs taken with a certain lens over others, that can inform which you pick up at the next shoot.
Also, if you organize the images on your hard drive by subject, like I do, it can be fun to filter by date to find pictures you took in August of 2011 or instance. Or even just go year by year to see how you've improved over time.
Create a Collection of Your Current Portfolio
When people ask to see some of your work, do you find yourself scrambling all over your Library to pick out the latest and best stuff? Well stop that right now. Start a new Collection called 'Current Portfolio' and when you finish an image that you really love, drag into the folder. Ever time I add something new I also take a minute to look through and see if there's anything in there that doesn't make the cut any longer and cull it. That way it's an always up-to-date reflection of the state of your portfolio. This also makes updating your website easier because you've already got your best stuff put aside.
Create Export Presets
Speaking up updating your website, here's a time and headache saver. Say your site is designed for 1000px wide images with a small watermark, add a user preset inside of the Export dialog to make it easy to create resized jpegs for your site whenever you need them.
You can sharpen (I always export stuff for the web with the 'normal for screen' settings) and even point to a specific target folder. For example you could have an 'Full-res to Dropbox' preset that just saves an 85/100 quality jpeg over to a 'Finished Images' folder in your Dropbox. Use that setting whenever you finish a picture and you've got another layer of backup in case your house burns down and your primary and backup drives are toast.