shayhowe.com
Tables should be used for layout, font tags for different typefaces (and sizes, if possible). If you must use CSS, put it inline. No form elements or javascript. And you need to make darn sure that everything looks good without images, because that's how the vast majority of recipients will first see it.
My understanding is that most email marketing systems track activity - including open rates - through systems built in to either viewing the image or clicking through the links. I might have many people reading my email without images, and therefore appearing inactive - removing them my improve my stats, but not my marketing outcomes!
On the other hand, your email could be being spamboxed (or worse, blackholed) because half of your list has marked you as spam, and you're continuing to send to them once a week.
There isn't really an easy one-size-fits-all solution to that. Using a decent email service provider is a good idea, if you have the budget for it. They'll be able to give you a lot more information about your email's performance, likely have better delivery rates than your current mailserver (assuming you pick a provider that doesn't suck) and, importantly, will be able to identify and remove recipients who have marked your email as spam.
Edit: Deleting bounces, on the other hand, is extremely important, particularly if you have lots of webmail addresses. Possibly the most important thing you can do to maintain or improve delivery rates. That really should have been higher up on the list!
The way we do it is to identify the people we want to drop first, then send them 2 or 3 blasts with increasingly dire subject lines and a huge "opt me back in" button before we unsubscribe them. Somewhere around 1% of people opt back in that way -- even if they haven't loaded images or clicked a single link in years. My impression is that it's mostly people who are reading the message on a crummy old Blackberry or similar device with poor HTML support.
Remove your bounces. I cannot stress this enough. If you're sending using a homebrew system or a script on a webserver or what have you, make sure it can process bounces. Excessive attempts to deliver to dead addresses is the number one metric used by large email receivers to assess your mailserver IP's reputation.
Keep your list clean. Don't be tempted to add those 150 random names and addresses you found from last year's marketing push. It's not worth it. One or two bad records on your list is enough to completely decimate your delivery rates. Stick to double opt-in and don't be tempted.
Make your unsubscribe link obvious. Really obvious. Like, at the top of your email, or in big black text. Too many people feel the need to hide their unsubscribe link away in tiny text at the very bottom. If someone doesn't want your email, they have two choices - spambox it, or unsubscribe. If many people spambox your email - your delivery rates will suffer. Make unsubscribing easy and obvious.
Not an easy one to test, though ;)