How Can I Clean Invisalign?

Normal invisalign use is 2 weeks and they should really not get dirty.  Daily rinsing and brushing the trays would be best  although as you stated they do have crystals to purchase.

If you take care to CLEAN your trays DAILY with a toothbrush and mild dish detergent, AND be sure to brush your teeth after every meal before re-inserting your trays, they should stay clean. As you seem to know, there is a soaking solution available thru Invisalign, as well, but if you follow the above, you should not need it. I assume you are talking about your aligners, and not your retainers!

Yes those trays can get pretty yucky if you don't clean them properly even after a couple of weeks and especially if you drink or eat with them in at all.  Use a toothbrush and toothpaste.  Some suggest a little soapy water an then rinse well.  A little dilute bleach is good too.  Finally, you can also try denture cleaner.

I learned this in dental school...we used this really neat and cheap method to keep dentures and now Invisalign trays clean.  What do we use for white clothes moms?  We use bleach...yes and that works for Invisalign trays too!  At night, after you have cleaned your trays with your toothbrush/toothpaste, mix a small amount of bleach in a small amount of water and drop in the trays for no more than 30 seconds. Voila! you will see all your stains lift.  Your trays will feel so good when you put them back in your mouth.  This is OK to do every couple of days.   Enjoy!

Since the trays are only used for 2 weeks, they normally don't get too "dingey".  Simple scrubbing with toothpaste is normally enough.