Perhaps the lose of erection could be down to him worrying if you are enjoying the moment. Maybe try it as a 69 activity and see if that makes a difference. Alternatively, talk dirty whilst you do it, or if your both happy to. Get your husband to film you performing oral. You can always delete the video straight away.
I find I can get bored giving a blow job so I try and change up my technique throughout. As in using my hand and mouth, deep throating, using just my tongue, using my tongue to lick his head on the way up/down. The hardest thing I find is when I want to swallow the saliva building up because I have to momenterily stop to swallow, but from experience I've learnt the wetter the better when it comes to giving head and nobody seems to mind the streams of spit running down my chin/their balls :)
I would reccomend this http://www.lovehoney.co.uk/product.cfm?p=21160 for getting used to deep throating, a weird sensation at first but no gagging involved :)