MacOS – Best practice for installing applications for multiple users

I have a MBP with Lion (10.7.3) on it and there are two user accounts on it, I use them to seperate work and private stuff, but when I install apps, I generally want to use/upgrade/remove them using either account.

Applications that come as a package (.pkg/.mpkg) tend to install/upgrade easily and require admin permissions in a user friendly way, no problem here.

Other applications just come as an app bundle, which you can move to a desired location, in my case this is the /Applications folder, since I like all apps to be in a central place. But when user A moves an application there, there's no need for a password, but a application will be owned by user A, and read-only to user B. A lot of things that are supposed to be user-friendly break in this situation, for example: applications that upgrade themselves stumble over this, and instead of replacing an old bundle with a new one, I have to manually delete the old one before putting the new one in place. It's not the end of the world, but I wouldn't exactly call this user friendly.

So, how is it possible that user A can dump something in the /Applications folder without using a password, only to find it read-only to user B? It's very annoying. Also when I install something in account A, but use it the first few times using account B the "this application came from the internet, are you sure you want to use it?"-warning doesn't go away until I use it with account B.

I'd like the applications (or their folder, whatever) to be read-write for both or writable using a password or something. Is there a way to make this annoyance go away? Should I install these applications in a different way? I want to manage my apps "user account-agnostically" if that's possible.

I have experience with linux/unix/command/BSD subsystem, what have you, so answers of a technical nature will help. Of course I have tried chmod, chown creatively to tackle single instances, but I don't like fiddling around with permissions when I'm in the mac's natural habitat (system folders/app bundles). So any info on this matter will help.

EDIT:
Just to make it clear, I would like a nice workflow for managing/upgrading apps between two admins on the same computer, it shouldn't require an understanding of file permissions as it does now, it seems this situation is something the developers at Apple never thought of. Filling in a password is no problem, but sometimes deleting an app (in order replace it with an upgrade) is impossible unless I open Terminal and use sudo rm -rf /Application/SomeApplication.app. You just can't expect that from two users of a single computer that know both are able to install/delete apps but aren't technically savvy.