Maybe the most important part of creating a landing page is organizing your landing page theme. How you select your theme, of course, will all depend on how you plan to generate traffic.
If you decide to generate traffic through search engine optimization, planning your landing page theme will entail finding phrases within your niche which have a high demand (aggregate search value) and a low supply (small amount of competing sites) and then creating multiple landing pages, each which is optimized around a different phrase.
If, on the other hand, you decide to generate traffic through pay per click (PPC) programs, such as Adwords, planning your landing page theme will again entail tuning a number of different pages to fit the keywords you are purchasing.
This is actually where most people fail when they create a landing page: they don't tune it to fit a specific audience. For instance, in the case of a squeeze page for a newsletter, they might start a newsletter about toys, but they only create one landing page and send all traffic to it. This is a big mistake.
Chances are, if you create a quality product or newsletter, it can benefit a number of people. So why not communicate the exact benefits they will derive from subscribing or buying? If, for instance, you have a newsletter about legos and toy blocks, so you group it under the loose heading of "toys," a visitor who is looking specifically for information about either legos or toy blocks will click off your page if they don't see the direct connection to the exact topic for which they were searching.
Instead, you will want to setup a page centered around legos and a page centered around toy blocks. On each page, you will want to communicate the specific benefits to joining the list for each of those groups of visitors.
Going one step further, in addition to organizing your landing page theme, if you are creating a landing page for a newsletter, you may also want to segment your list, so you can send information specifically about legos to those who request it - and information about blocks to those who request it.