If you hate using chemicals to get rid of greenfly then try using washing up liquid mixed with water. Then spray onto the infested area. Another way is to remove the aphids by hand on a regular basis.

If you have apple trees or you are growing broccoli, plant Nasturtiums near by as this will help keep aphids away.  Planting Nasturtiums in the greenhouse will prevent whitefly infestation.

If you have aphids attacking your fruit trees or roses, try planting garlic near to them, as the strong odour of the garlic repels them.

 One of the greatest enemies a gardener fears are slugs and snails.  Slug pellets are effective but may contain chemicals! Below are some great tips on how to deter & destroy them.  
 
For the slugs, go to your local barber or beauty shop. Ask them to save the hair they've clipped off their customers. Spread the hair around your plants. The sharp cut ends of the hair are a devil on the slugs' soft little bodies.
   
 Protect your potted plants from slugs & snails by placing mechanics grease half an inch below the top of the pot all the way around. Or better still use WD40 lubricant .
 
 Crushed eggshells are bad on the slugs, and the shells also add nutrients to the soil. Birds like eggshells because they add calcium to their diet so the eggs they make are stronger, and if a bird is eating eggshells happens to see a bug, well you can imagine what will happen to the bug.
 
 As you know slugs love to party at night so why not treat them by supplying the drinks. Place a pint glass with some beer in it on it's side where they hang out . The next morning you will find lots of dead slugs , but at least you gave them a good farewell party. Birds will eat the corpses and be will be singing merrily all day long.

Another approach is to use a bed of sand around your plants. Slugs and snails do not like coarse materials and will not cross it. This has limited applications. But, it works well with fruit that lies on the ground like pumpkins and melons. Gently lift the small fruit,and lay a bed of sand so there is at least a three to four inch perimeter of sand on the ground around the fruit. Add more sand as the fruit grows.

A Note on using Salt: Some people get a kick out of pouring salt on slugs and watching them melt. Yes, it works. But, it is not practical in your garden. Too much salt in the soil is also fatal to your plants. Use only on paved areas.