what makes MU plugins work? I'm looking to port some non-MU ones over.

I have two plugins designed for the standard WP. I need to hack them & port them to MU, 1) for them to function correctly & 2) to allow them to remain activated on every secondary blog. For some reason, even when using Aaron's plugin manager, they don't stay active on secondary blogs... and I need them to 100% of the time.

Are there any tricks to writing *good* MU plugins? I could use any tips to ensure compatibility.

One plugin has a single-field form in an individual dashboard page that simply allows users to upload an image that is automatically placed in a specific location on the page (in this case, a logo). I assume a hack is needed here to upload to the appropriate /blogs.dir subfolder.

The other adds some form fields to the standard "write post" page. It allows users to add extra data (which then gets placed similarly as with the aforementioned logo plugin) without messing with custom fields/keys, which helps my site's usability for non-technical members.

Any suggestions would be welcome. These aren't complex plugins but I need them to work flawlessly with MU. Thanks!!