I spy on my readers: I love checking "my Statcounter" to see who's reading the blog, and what they're looking for when they get there. People often stumble on this site because they were searching for how to install a power outlet or strip paint using a heat gun. I always think that's pretty funny, and I love the idea that there's actually someone idiotic enough to take home improvement advice from my blog. They deserve the electrocution and subsequent house fire I'm sure they ended up receiving.
Here's an interesting search field that brought someone here recently, though: "intonation for beginning orchestra". That's something I've gotten much better at teaching over the last year or two, and I'm starting to hear some good results from students who couldn't previously play in tune.
If you're am elementary string teacher, I advocate for a multi-sensory approach to teaching intonation. Activate their visual sense with reference points on the fingerboard. Encourage them to "feel" the placement of their arm/wrist/fingers as they play the note in tune. Get them to listen to themselves and the sounds around them. I refrain from the taste and smell exercises recommended elsewhere (my students aren't allowed to lick their instrument. It's a rule: they have to join band if they want to do that stuff).
The Visual - Yes, lines or stickers on the fingerboard can help alot; I tend to avoid them for as long as possible, though. I don't like the effect on the student's posture when they start to stare at the marks on their fingerboard: all the visual learners (constituting most of the population, from my experience) want to stare at the stickers or marks as soon as they get them, and are likely to forget all of the good playing habits they've learned. When students do that, I make them close their eyes and play slowly. At first, they seem to think they won't put the right finger down if they can't see it. They are usually pleasantly surprised to find out they can play the right notes to a major scale or "twinkle" with their eyes closed. Then they start actually listening to themselves. If you do this, it's a good opportunity to gain control of the pace off the lesson, reminding the student of all of the elements of good performance posture ("head into the violin", "wrist hanging down", "relax the shoulder" or what have you).
The Tactile - I try to get the kids to remember the feeling of their hands when in the right position. I don't know any tricks for doing this: I just tell them to close their eyes (if they are one of the very visually focused kids) and "feel" their hand. I always feel like Yoda when I do this. As an intersting side note, there is a professional orchestral mallet player who is totally deaf, but can play correct pitches by feel. There is probably a way to incorporate "feeling" the correct pitches into elementary string lessons, but I haven't gotten there yet.
The Aural - It seems obvious, but I think most young students need to be trained to listen to themselves when playing. They can do it given time and effort. It seems to work best when they're playing in unison with someone else (their section or lesson group, for example, but it's better if they can play in unison with an experienced player who will model good intonation for them). Encourage them to "sit on" fingered notes for a long time to really start to hear the "in tune" or "out of tune" quality of the notes. For years, I only had beginning string sections play long notes in unison on open strings (as a bowing exercise): I'm starting to see the value of playing long, sustained, stopped notes in unison as soon as possible. Have them play open d, sustained for the length of a few bows, then challenge them to "step up" and stop the e on the d string. Some of them will adjust their pitch right away, some will require encouragement and time. Tell them individually if they need to go higher or lower. Compliment them when they get more in tune. They do get it, given time.
One thing I've found is that it's a good exercise to play another instrument when playing in unison with students while focusing on intonation. If I play cello, bass, or piano (or sing, for that matter) with an upper string student, they spend less effort trying to copy my technique and listen more closely to the notes and phrases. Obviously, one should model playing technique for them initially, but then the result can be surprising when you let them go off on their own.
A caveat: This stuff only works effectively with the bow. I know there are many school orchestra teachers out there who start the kids off playing only pizzicato: in my opinion, there is no way the students will develop any idea of tone or intonation doing this. They have to use the bow from day one.
There are lots more interesting things that can be done with second or third year students (or beyond): they can develop more independence through playing rounds or harmonized parts, for example. However, this will work best if they've already developed a good strong foundation in playing "in tune".