The best writing tip I can give anyone is to find themselves a writing buddy/critique partner - it's been invaluable to me and I honestly wouldn't be published without the support I received from my writing buddies.
It can take a while to find the right one - and do take your time as it's a partnership that is probably going to last for a long time and it has to work - but if you persevere and connect with exactly the right person, you'll never regret it.
A writing buddy should be at more or less the same level as you (so if, for instance, you're new to writing, she/he should be too) and they have to like your type of book, even if they don't write in the same genre themselves. I have two critique partners, one of whom writes the same sort of thing as me and one who doesn't, and that's worked out really well too as I get two completely different takes on my stories.
Your WB isn't just there to read through your manuscript when you're finished though. She/he should be with you every step of the way - someone to brainstorm with, bounce ideas off, try out scenes on, a shoulder to cry on when things don't work out and, most importantly of all, a cheerleader for when things go well. In short, your WB should be one of your biggest supporters, as you are for them!
This partnership has to be based on trust and respect. You must like the other person and his/her writing and feel sure that they know what they're talking about when they send you critique, especially as it might sometimes be something you don't really want to hear (even if you can see that it's true). Our manuscripts are precious and very often we don't see the flaws - that's when we have to trust our WB to see them for us. Naturally you can disagree, in the nicest possible way. As the author, you always have the final say and you may have certain reasons for putting in a scene your WB wants you to take out. At the end of the day, it's a sharing of views, a discussion, which should ultimately make your book the best it can be.
So if you haven't got a writing buddy already - go and find one right now!