Research.

Firstly, have you researched the 'demand' for this item? If it is entirely 'new' to the market (i.e. performs a function previously unheard of) have you distributed it to impartial users for constructive feedback?

If your device passes the above then you have to be brazen enough to go knocking on doors.
Door knocking can be as simple as a letter to interested parties - which will likely be one letter of many that they receive and therefore gets little/no attention or you can use the 'direct' approach.
'Direct' could be anywhere from your local bank (the least likely to help given recent well-advertised attitudes to the public) right up to the Managing Director of the biggest manufacturing company you can find locally - nationally if you're that convinced it's a good idea.
According to various reports, making appointments is perhaps the worst way to go about this and arriving on the doorstep unannounced and being quite insistent about seeing someone may be the required action to take.

The hardest part is being ready to accept rejection. Not all recipients of your device will either understand or appreciate what it is or what it could do or offer unless you are very specific about who you approach. 10's or 100's of rejections doesn't necessarily mean your idea is a dud but you do have to prepare for the fact.

Make your presentation simple, direct, succinct and be prepared to leave a concise package/sample for them to test at their leisure. It's easy enough to protect yourself and your idea by retaining the coding and securing copies of your 'package' with a solicitor or other registered keeper of documents.

TV programs have amply illustrated that the boldest are the most successful. If you don't have the bottle to do 'all of the above' be prepared to show someone who IS and let them act on your behalf.

No-one ever said that success comes easily or cheaply.