This page provides a guide to some of the best-known remedies available to today's gardener.  Following that, is a guide to companion planting; another environmentally friendly tried and tested method of pest avoidance.
I have only included Organic methods, as that is the way the Gardeners of today seem to prefer.  Your vegetables and plants are ‘Slug paradise.’  Wet winters combined with temperate, wet summers provide the ideal conditions for Slugs.  Their natural enemies, weather-wise, are frosty winters and hot summers.
The most common Slug is (Arion hortensis) or the Garden Slug.  It is grey or brown, with an orange sole, and around 4cm (1.5”) long.  Another Slug you may encounter is (Derocerus reticulatum), or the Field Slug.  This is distinguishable from the Garden Slug by distinct brown patches on its sides.  Both are bad news for your tender plants and vegetables.
The Black Slug (Arion ater) is another you may encounter.  It can be up to 15cm (6”) long and is called the 'Black Slug because of it's colour, although it can be brown, brick-red or orange.  There are even albino Black Slugs.  Many people assume that this is the Slug which is doing all the damage.  In fact it causes very little damage to your plants - it's the much smaller Garden and Field slugs which cause all the damage.
There is a less common Slug known as the Keeled Slug (Milax budapestensis).  It is about 10cm (4”) long and has a definite ridge down its back.  Whilst not as common as other Slugs, it does cause considerable damage.
The latest and possibly the most effective way of controlling Slugs is the use of Nematodes.  They are tiny organisms, so small they are invisible to the naked eye.  They are naturally occurring organisms which are totally harmless to you, your children, pets, wildlife and your plants.

They are available to buy in plastic packages, and you just put them into a watering can, add water and then water the areas affected by Slugs.  The little nematodes then enter the Slugs and release bacteria which slowly kills the Slug.  The nematodes then multiply and go in search of more Slugs.  Farmers have been using them for several years and are increasing their use of them each year, as they are proving to be very effective.
Many Gardeners scatter sharp material around their plants to prevent the Slugs getting too near them.  The most popular products used are soot, sand, ashes, bran and broken egg-shells.  These definitely make it considerably more difficult for slugs to get near the plants by drying up the mucus which they rely on to move.
All of these substances will be affected by wind and rain, so frequent re-application will be required.  They also suffer from the disadvantage that they do not lower the number of slugs in the garden, although it may well encourage them to take up residence in your Neighbour’s garden instead of yours!

If you want to protect a few plants you could make a circle of garden lime around the plant about 3cm (1”) wide.  This seems to stop Slugs crossing.  Another similar tip along this line is to use dry Holly leaves or large amounts of pine needles to surround your plants.
If you find Slugs and Snails in the greenhouse, and the greenhouse is empty, try a Sulphur candle, then do your best to seal all the gaps to avoid them coming back.  Alternatively, plant some Lettuce or similar that you are willing to sacrifice to the Slugs in the hope that they leave your preferred plants alone.

If you use trays on staging in the greenhouse put the legs of the staging in a bucket and half fill with water – it is generally thought they can’t swim!  Keep the staging from touching the glass and you hopefully will have created a safe 'island'.
The 100% barrier method that works for seedlings is a plastic bottle with the base cut off.  Insert it at least 10cm (4”) deep into the soil around the seedling, and only the most determined Slug will succeed – it makes a great cloche as well.  The problem comes when the seedlings outgrow the bottle.  However, Slugs really do prefer the tastier younger plants.

Another alternative is a band of flexible plastic placed all around a group of plants.  These are available from Garden Centres. The plastic should be at least 30 cm (1’) high; 10 cm (4”) below ground and 20cm (8”) above the ground.  This will not stop all Slugs, but will deter most.  The only problem here, is that slugs may already be inside the plastic wall, and they will be unable to get out.
Quarter fill a jam jar or any plastic container with either milk or beer, and sink it into the ground so that it is about 1 to 2cm (1/2 to 1”) above the soil level.  The Slugs can easily climb this, but it will stop Ground Beetles from entering them and drowning.  This method definitely works, although you need at least 4 jars per m2 (yard), and they will need to be replenished every two or three days.
This is easily the most effective method.  Simply collect the Slugs by hand (tongs, or a long needle on a stick can be used if you don't like touching them) and transfer them to container of salt water.  You will need a torch to spot them - expect to kill roughly a hundred per hour.  Search on the lawn, paths and around tasty plants.

You may well find that a large number of Slugs are on your lawn at night, rather than directly on the soil.  This raises the interesting proposition of getting the lawn-mower out at midnight and single handedly massacring thousands of Slugs in half an hour.  Personally, I have not tried this method, but I have been assured that it works a treat!
It is generally considered that if you take a Potato, cut the edges off to form a cube without any skin, then push some form of skewer into the potato and then bury it into the soil.  Hopefully the Wireworms will make a beeline for this, leaving your spuds to grow quite happily.  The skewer is so you know where the test Potato is and for easy removal of the Potato.  This method sounds like a long-shot to me!

Rhubarb spray is generally considered effective for stopping leaf-eating insects.  It may be too much effort for one plant, but it will store for the future.  Although Rhubarb leaves are poisonous, they can be used to make an effective, organic spray that will kill leaf-eating insects (and hopefully Black Flies too) in your garden.  This spray is harmless to Bees and breaks down in the soil quickly.

Simply boil a few pounds of fresh Rhubarb leaves in a few pints of water for 20 minutes.  Allow the liquid to cool and strain off the liquor into a container.  Be sure to use old utensils as this will stain and poison the pot and the strainer!  Next, dissolve approximately 100g (4oz) soap flakes into the mixture while stirring it vigorously.  Pour the resulting mixture into a spray bottle and apply to the infested plants.

Whilst a physical barrier is indeed useful against things like Carrot Fly and the like, companion planting can go a great way to confuse your average flying pest.

It is generally considered that they use smell as a way of finding the plant they require.  The idea is, you just need to confuse their sense of smell.