Missy - there is a lot of great advice here so far, especially from Steven Mason and David Richard.
Some additional comments to your specific questions:
1) What works well in a pitch: Subject line is critical. In the pitch itself, the rule of thumb is to open with the punch line, and then tell the joke. Lay out what you have to offer/say, and what the ask is (news coverage, demo, etc.). This first sentence should be equally concise and compelling (that's the magic). Make the rest of the pitch easy to parse at a glance, to make the important stuff easy to find -- don't bog it down with long paragraphs.
2) Exclusives: As a startup, it is rare that a reporter will write a story based on your pitch alone - if it will be supported by an announcement. The press release offers a lot of the supporting detail, a formal commitment to share the story, and a dateline to force the urgency. However, in most cases I would advise against sharing a full press release, unsolicited, on the first email - especially if there is no prior history with the reporter/editor. Instead, offer the press release and other info (depending on your offering - it could be an advance look at a product or app) if they are interested. This helps establish an active dialogue, which helps you gauge genuine interest.
Start by offering to share the announcement in advance of the release date - this is the "embargo" scenario. Unless you are going after your clear #1 or #2 target (as outlined by the advice above), never open with an offer for an exclusive. This immediately limits who you can pitch it too -- and if the story falls through, you are left with nothing.
The press know you want to create awareness and are likely pitching it around -- some may require an exclusive to write it (TC and VB often require this due to the volume/pace of deal flow news), and others are OK with an embargoed date. But always be honest, if you shared the pitch or release with other media, you need to reveal that (not who you pitched, just that you have...if/when asked) and give the reporter the chance to determine if they can pursue the story - since they have a managing editor to answer to. Otherwise you will burn them, and a bridge.
At Resound, we always create an "exclusives" strategy for a client before any news-driven campaign. Determine beforehand which outlets would be worth an exclusive story, so when it comes up, everyone is on the same page on the decision -- because it generally needs to happen quickly. If an exclusive on the full story is not in the cards for your company, perhaps you can offer some exclusive story elements as a response -- an interview with a well known customer, reserved JUST for that editor. This way, while they may be covering the same news as another outlet, they are assured a fresh story with access to a resource nobody else was offered.
Happy pitching, and good luck!