Ascend Charlie's method is not only good for a chopper, it works well in any aeroplane when you are stuck low down below cloud if you do a bit of preflight preparation with your chart. It's really just a variation on a theme long used by mariners when in coastal areas.
Look for features on your chart along track that may be lined up, e.g. a small lake with a prominent hill or small town behind it, and also look for features that will line up to form these so-called 'transit' lines cutting across your track at 30 to 90 degree angles. Rule them in on your chart in pencil so that they cross each other. Not too many as you don't want to get confused, just obvious ones or ones you hope will be obvious. Where transits intersect is exactly where you are and even if you can't get them to line up conveniently with your track, you will soon get the idea that any transit, if properly identified, is as accurate as any radio-aid derived line of position.
Don't panic if you occasionally can't see what you expect once in the air - some features on WAC charts just don't show up too well.
Keep ticking off the 10nm marks as well, and if you miss one, just keep the DR going.
Also, the advice to build in a deliberate offset is good, but take into account the sun - you don't want to be looking into the sun for your destination.