I'm a Canadian citizen. I'm planning to go to New York for a week (entering by land) and then fly from there to the UK to stay for three months (I have a valid UK work visa). I'm wondering if I might have any issues entering the US because I'll be travelling with a suitcase packed for a few months. I'm planning to bring a printed copy of my ticket for my flight from New York to the UK to prove that I'm leaving a week later, and my UK work visa is in my passport, but should I have anything else to make it easier? Something from school or from my Canadian workplace to show that I'm definitely planning to return to Canada and definitely do not want to stay in the States?
As a Canadian citizen, when you enter the US (especially by land) they'll probably ask you simple questions like "Where do you live?", "Where are you going (in the US)?", "For how long?" and then if you give sensible-sounding answers, they'll send you on your way. Certainly going to New York for a week on the way to the UK is nothing out of the ordinary and they're unlikely to suspect you of anything.
If any harder questions come up, then you've certainly got sufficient evidence (ticket, visa) to back up your statements.
The job of the border guys is to make sure they're confident that you're not trying to 'sneak in' as you suspect they may be concerned.
I'm not sure which airport you're flying in, but often the suitcases is after immigration anyway (much like Vancouver, if that helps you imagine it). However, if it's not, and they do question it, it's simply to assure themselves of your story.
So, to satisfy this, bring documents. Evidence as you said, of your ticket, your work visa, and if you can include say, your accommodation booked (or a contact you're staying with). If you have accommodation in Canada that you have evidence for, that'll be fine too.
Generally though when I've gone through the US (NZ citizen), twice with a LOT of (oversize) luggage, they've simply checked the visa, and asked for my onward flight / bus ticket. I got far more questions about where I'd come from when I flew in from Bogota...