My best practice suggestions:
1.) Search First: If the underlying material is something that may have been discussed in the past, do a search. If you have questions, they may already be answered. Old posts may already have comments relative to your discussion. If you decide that the thread is useful and you are going to Necro it, start your 'Raise Dead' post with something that acknowledges that you're necroing the thread, such as, "Mordenkainen's Tome puts a new spin on this old thread ..." Even better, if you started the old thread, consider tweaking the title to show that it has been necroed.
2.) Referencing is ok: If you think the situation has evolved and it'd be better off to leave the old thread dead and start a new one, it isn't a bad idea to reference the other thread and provide a link. That will reduce the chances of an (annoying?) one sentence post telling you it was already discussed with a link to the old thread. Odds are someone will reply to the old thread, however, so you may end up with parallel discussions on your new thread and the old one, so this is a judgement call.
3.) Don't take offense: If someone comments that you're necroing a thread, don't assume you're being chastised. It may just be someone noting their surprise to see a topic reinvigorated - not a reprimand. I remember not too long ago where I started off my reply in a recently necroed thread by saying something like, "Wow... this is a really timely thread necro." I meant to convey that the changes from Mordy's made the topic relevant again and I was glad someone brought it up. The Necromancer was offended because he assumed I was chastising him. (On the other hand, some of us are insensitive and rude louts that may be giving you the efinger unjustly. $#@! them. They're not worth your worry. There is a wonderful block function on this forum.)
4. Consider whether you're the right person to post: If you're going to post a topic that a lot of people care about, such as news of a new WotC release announcement, a UA article, or discussion of a popular component of a new product that just hit the shelves: Ask whether YOU should be the one to post it. I won't post these things because when I do so, I cut out members of the community that I've blocked - and I've blocked a few members of the community that want to be involved in these discussions. Rather than start a conversation they're either entirely excluded from or that they'll duplicate so they can talk about it, I let someone else start the discussion. There are no points gathered by being the person to post the topics... and I think that things that will live as News Items are better posted by Enworld folks with the access to make it a news story easily when it is their post, but for which it is harder if it is someone else's post.