I confess another of my hats as a yacht instructor has given me lots of insight into good and BAD foul weather gear and it's just as applicable to cycling as the high seas..although lightweight gear is not a sailors' prime issue.
Firstly, make sure you avoid natural fibres such as wool and cotton and old fashioned gear like wax jackets - they are all great at retaining moisture from sweat and rain and eventually will all make you cold and miserable.
Secondly, don't be fooled by labels - good gear is expensive, but the top brands are definitely not always the best for the job. I wear expensive waterproofs at sea, but spend many days afloat so it has to be hardwearing. I equip students with breathable, light, but cheaper gear and it works just as well, but we have to replace it more often.
Thirdly, layer up to wick the moisture away from your body. Using thin layers means you can remove them as you get warmer and replace them as you get colder again. There are many polartec fleece and other lightweight breathables available from even the cheapest of high street stores.
Finally, the top coat, waterproof layer, needs to be just that, and breathable or you will be back inside that bin liner again. Goretex based fabrics are still the top line products, but other lower cost fabrics do also work. Shop around and make sure you find something comfortable that allows the full range of movement you need.
Happy cycling and I hope everyone heading to Presteigne tomorrow has a good set of waterproofs packed!
Richard
Richard Beniston
Pedelecs UK