Always create additional ads within the ad group if you want to try new things.
Also, you should really learn how to utilize the different matching options Adwords offers.
[adwords.google.com...]
Take a look at the Keyword Matching Tutorial.
I never remove keywords because of the lack of impressions. You've already done the work, why not leave them active?
Rule #1: Don't mess with a well performing Ad
The reason to not mess with ads, is as soon as you change any single thing on it, it goes back into review and stops showing on content and search partner sites.
Make a new ad (or several), wait for them to be syndicated, and then have a little patience while they get enough impressions to see which one works. Using the ad optimizer can be a great feature for letting G determine which ads you should use.
[webmasterworld.com...] is a thread I wrote earlier this year which I think explains things nicely for newer AdWords users.
So, I'd say tweak away! I would advise, though, that you make informed decisions and then closely monitor the results. And I very much agree with Robino and eWhisper about not editing an ad that is doing well, and creating a second ad instead.
One other quick comment regarding the following quote:
I never remove keywords because of the lack of impressions. You've already done the work, why not leave them active?
There is a potential downside to keeping keywords that get no impressions over time (or which are otherwise not performing well for you).
The downside is that, in cases where your daily budget is lower than recommended, your ads will not be showing all the time. And in such cases, if you have a bunch of non-performing keywords in your Ad Groups, your ad will end up appearing less for the keywords that you really care about.
This is because all those non-performing keywords are still being budgeted for by the system that calculates your daily budget. If you get rid of non-performing keywords, they are no longer budgeted for, and your daily budget will then 'go farther'.
In other words, your remaining keywords will show more often for the same budget.
Hope that makes sense!
AWA
I felt, that happend in my campaign when I created a new ad group and paused the older one. Finally I was desparate about it and deleted the older ad group.
