I've used custom made plugs, with motorcycle specific filters and they were fine at first but did have some problems.

They would just touch the helmet lining (Aria Tour-X) and I'd get a vibrating noise from the wind on the lid,
After 4 hours or so they would become uncomfortable and I'd have to swap them for foam ones.
Music would sound odd, as some of the notes would be blocked by the filters.
Eventually as the ear canal changed (which it does over time - apparently) they became pretty useless and I switched to SNR30 corded foam plugs that cost a few pence each (if you buy in bulk). Wear them for a day/ride then dispose of them and open a new pair. There is a nack to fitting them, but once you've got it it's easy.

I've tried many types over the past few years and found that any with hard centers or parts that stand proud of the ear cause irritation where they touch the helmet lining - so the foam ones are firm favorites

Having used numerous types of earplugs, plus a professionally moulded pair that got too uncomfortable after an hours use, I can highly recommend these.

http://www.howardleight.com/earplugs/max

Very soft in the ear. I also use them when sleeping off nights.
I buy mine at the local bike shop but you should easily find them on tinterweb.

Thanks for all the good advice from everyone on this subject....there doesn't seem much to recommend the expensive custom built ones....for �120 ish I would want perfection! , seems to me they are too easily dislodged. I bought some reusable Auritech one's today, similar to the Pinlock, for �20. And a box of Hero's foam plugs turned up that I'd ordered online and forgotten about. ...I have ear plugs literally coming out of my ears Steve

If you don't like having things pushed in you ear like myself then heres a cheap mod for your helmet to quieten things down a bit on long stretches on the bike. Take 1 ( preferably washed ) sock folded neatly & place it in the ear void of your lid.

Hi Desert Racer. ...are you telling me to put a sock in it...? Steve

Yup I did, it was a good test today as it was pretty windy on the way to work this evening. The socks cut a lot of the noise out. I would recommend ankle socks as any longer fills the air gap to much then squashes your ear.
You may or may not also wish to know that I cut up some boxer shorts ( never worn ones ) & tucked it in under the chin of my lid to stop the wind blast that comes up through & dries out your eyeballs after a while. I'll try posting a picture by phone while I'm sitting here waiting for a barge.

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The best cheap foam ones I've found are the Moldex Spark, I've done lots of research, trying loads of different ones but these seem to suit me the best, they squash up really thin making them easy to put in, and seem to stay in!