Use the same careful techniques you use on the front element. Camera companies are very aware that the front and rear lenses get dirty and need to be cleaned. As such they won't but fragile coatings or soft glass compounds in those elements. What you're doing is perfectly good and safe to do. Here are a few tips per your request:

Tools of the trade:

• Air blower
• Lens brush
• Microfiber cloth (nothing cheap or it will probably leave micro lint everywhere)
• Methyl alcohol wipes

Smudges:

1. Alcohol wipe - one to gently lay over and pick up any dirt/sand
2. Alcohol wipe again to gently rub
3. Microfiber cloth

Dust:

1. Alcohol wipe - one to gently lay over and pick up any dirt/sand
2. Air blower
3. Lens brush
4. Alcohol wipe
5. Microfiber cloth

Sand:

1. Alcohol wipe - one to gently lay over and pick up any dirt/sand
2. Air blower
3. Lens brush

"Slime": I tend not to slime my lenses, but if it's more like ectoplasm than pond scum I suggest the following:

1. Soft silicone spatula to scrape the bulk of the slime off (careful not to scrape the lens)
2. Microfiber cloth to remove what is left of the heavy slime
3. Alcohol wipe to cleanse the fine remaining slime
4. Air blower to dry out the slime
5. Alcohol wipe again
6. New microfiber to clean what's left