For sending pics, you have to ensure that they're lower resolution - MMS has a size limit that varies by carrier, but it's not much in any case. For full-res pics, you'll need to use email or upload to Flickr or Picasa or the like and then send a link to the picture.
Same thing on vids - they have to be short and lower res to fit within the size constraints. A normal high res vid can quickly hit several MB - way too big for an MMS. In that case, upload to your YouTube account, send a link to it via SMS.