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Here's a general outline of things to consider. Parts of it probably won't apply, and the order of various tasks would need to change depending upon what's already done/established, how the company approaches planning, etc.
Prior to Site Sevelopment
Objectives
Step back. What is your management expecting/hoping to achieve with the site? Take their guidance, whatever it is, get clarification on anything you don't think is not clear enough. Establish a budget if one is not there already. Then develop specific traffic and growth objectives, conversion objectives, ROI objectives, revenue objectives, and/or any other objectives that are critical to the success of your company. Get management buy in.
NOTE: Some of this may seem like overkill in a small or mid sized company. It's not. Going thru this process get's everyone on the same page, which prevents conflict and disagreement later, and it also is very valuable for all oars to be rowing in the same direction. If your company is not used to this, try to introduce at least some aspects of it. It'll help them, and it you do it deftly, it probably won't hurt your standing at the company either. ;-)
Research
IMO, every great site plan starts with clearly established business objectives, followed by a deep understanding of the category, which can only come from research. On the Internet, that primarily means keyword research.
Develop a list of kw's you want to target. The list should be divided into two parts:
With respect to organic rankings, often pages can target multiple related kw phrases and kw phrase variations. (Check out RobertCharlton's advice on how keyword development and page title development intersect [webmasterworld.com].) Also, check out the Keyword Discussion Forum [webmasterworld.com].
As for PPC, there are entire sites and books devoted to this topic. Suffice to say here that you should have a budget for this, and it should include plenty of testing to allow for early mistakes, experimentation and fine tuning.
Site Development
I doubt that there is any better primer on what to think about when developing a site than Brett's 26 Steps [webmasterworld.com].
Here are some other things to consider when deciding what you should be involved with, and a few items worth repeating:
Site Structure
The kw research should help bring into focus how the site should be strctured. Logical hierarchies will develop, along with cross linking patterns. Review these two classics on site structure when planning your pages:
File Naming
Page Development
Critical List.
To add a few things to the list... I've found that if a site has a history, there will often be server issues and multiple-domain/mirror site issues. All mirror domains and www/non-www variants should resolve to one form of one main domain (use mod_rewrite if you're hosting in Apache). Generally, I prefer the form...
http //www.domain.com/
(colon omitted to avoid live link)
...with "www" and with the trailing slash.
Make sure you never have the same page content appearing on more than one URI.
Also, the company may have multiple sites which that are not mirror sites, but which may still adversely affect each other, particularly if they share common vocabulary, interlink, and are hosted on the same server.
When you modify your site structure and page filenames, note that you may have to redirect old files to new ones. Check existing backlinks on Yahoo and MSN, and use 301 server side redirects to redirect existing links for those page to new ones.
If you're changing domain names, you may well disappear from Google for six or so months, so don't do it unless you have to.
In most corporate site redesigns I've seen these days, there's pressure to use drop down or fly out menu structures. If these get too big, I feel, they can provide visitors with too many choices, and they make it difficult to prioritize PageRank distribution.
Corporations are also tempted by whistles and bells like Flash, which slow a site down and/or make it search engine unfriendly. Lots of designers like to show off with visual effects as well.
Make sure that any design will load with scripting and Active-X disabled. Keep in mind that search engines don't execute scripts.
Dynamic sites can carry a whole raft of other issues.