I would give these tips:
- Don't make it all negative.
When people receive several criticisms at once, to their ears, it can start to sound like, "everything you do is wrong". Regardless of what your actual intentions are, people tend to extrapolate from criticism to a wider context, assuming that it is indicative of your views of their overall behaviour/nature/abilities/flaws - which can lead them to become quite upset and/or defensive. Include some (genuine) compliments or the odd bit of positive re-enforcement to show that you do value them. It's also not a good idea to speak in absolutes or with such certainty - this tends to really grate on people (eg. instead you could add in, "I think that it could be..." or, "Maybe you could try...").
- Remember that there is a difference between honesty and truthfulness.
You might speak in a way that totally adheres to the facts (truthfulness), but this does not make it the moral or decent thing to say (honesty). This isn't to say that honesty has no need for truth, it most certainly does, but openly stating an unnecessary, harsh fact, when it does nothing to improve the situation, is pointless as well as obnoxious. Try to consider how you can say critical things in a way that softens the blow and makes the information more palatable to the other person. It will make them more receptive to what you have to say.*
- Constructive criticism must be primarily for the benefit of the other person - not you.
As [MENTION=1206]cascadeco[/MENTION] said, it's not just an opportunity to bring up all the things about them that annoy you, they do wrong, or that you wish they would change about themselves. You have to ask yourself before saying anything, if telling them this piece of information will genuinely help them (and whether it is your real intention to do so). If it won't, it's just you unloading your frustrations on them, in the guise of constructive criticism (which is incredibly obnoxious and manipulative). If you do want to discuss something about them that is bothering you, make sure you address it as such, and don't try to convince yourself that criticising it is an act of kindness. If you want to do this effectively, remember (that unlike constructive criticism) it is you voicing your issues, not theirs', and accordingly it is better to use "I" and "me" language to describe the problem, rather than "you" (eg. "I feel like..." or, "It's hard for me..." or, "I need...", instead of, "Why can't you..?" or, "You are always...") - this makes it much less confrontational.
*Hypothetical example: You are in a shop with a woman and she tries on a pair of jeans, which clearly make her look rather fat. She then asks you what you think of them; you shouldn't say, "they make you look fat"; instead you say, "I don't think they really suit you". The latter isn't a lie, it's just a more digestible form of the truth.