Bands: if you want to get signed, you will need to send promos. First you will send them to labels, which will sign you eventually. Then you will send them to media, including record reviewers. Labels: you will do the same. Promoters: ditto. So how do you send a promo in this day and age?
The current standard, which thankfully does away with piles of physical promos, is to send an Electronic Press Kit (EPK) or Digital Promotions Kit (DPK) which mean roughly the same thing. Your EPK will be a zip archive containing your release in MP3, photos and a press release/biography. Each of these parts offers its own challenge.
MP3s should be of a decent bitrate, usually 256k or Variable Bit Rate (VBS) equivalent, and should be tagged appropriately with band name, album name and track name correct and consistent. The MP3s themselves should be in a folder within the archive named Band Name – Album Name. This enables writers to extract it completely and view the files as they write. If you are using Exact Audio Copy or a similar program, settings allow you to specific correct tagging by default. I also recommend installing Windows Media Player 11 and using the Fraunhofer MP3 codec which is superior to the LaME codec which tends to make heavily distorted music sound plastic. I use the following naming scheme in EAC:
%artist%\%artist% - %albumtitle%\%artist% - %tracknr2% - %title%
various\%albumtitle%\%albumtitle% - %tracknr2% - %artist% - %title%
Press releases should fit standard format, which is an entirely different article, and include full contact information for the label and promotions agency. If you include band contact information, people will contact the band; if the band does not reply, do not include this information and have them contact the band through the promotions agency. Include the biography in here, generally a paragraph or two but not more. Also useful to include are all band public sites such as Facebook where the band might post more images or information as needed.
Images should include at least the cover art and a band photo, but many bands include logos as well for use as headers. These pictures should all be large (800px+) and in a format such as JPG or ideally PNG, which is not lossy like JPG.
Once you do all of the above, put the files into a zip archive and make it available to reviewers. Many use services like Haulix to make it easy to limit downloads to specific reviewers, and to embed codes in the work which identify who each package went to in case it gets leaked. In my view, labels should worry less about piracy from this angle because the release will inevitably be pirated and your only real recourse is to DMCA search engines to keep it out of the listings so that people are encouraged to buy it or stream it through a licensed streaming service like Spotify, Pandora or Last.fm. For this reason, lately more labels have simply been linking to their DropBox folders in acknowledgement of the impossibility of stopping all leaks, and it takes just one to spread it everywhere — or someone at a record store who drops the album into a computer for ten minutes to rip and upload it, as seems to happen in Russia and Latin America much to the delight of downloaders worldwide.
The above is written in the hopes of receiving fewer incomplete or botched promo packages. As stated elsewhere, most labels spend little time on getting these right because they want reviewers to spend as little time on the music as possible, and because the people who write the reviews the labels will republish are those who are making a personal connection with staff at the label in hopes of future hiring or collaboration. But for a starting band or label this advice may be helpful.