What you know now that you wish you knew then...

That there are affordable entry level tri bikes at a similar price point as road bikes.  For what I paid for my Trek 1.2, I could have bought a Motobecane Nemesis from bikesdirect, which would have been a better starting point for me.

Really only at it for 2 years, but my tip relates to running.  I've run "casually" for years, but when I first got interested in multisport, I got all weird about running.  I became obsessed with heart rate and lactate threshold and all the stride fads that will eliminate pain or make you faster.  Pffft, forget it.  Forget all of it.  Get some good consultation on shoes and run.  That's it.  I quit obsessing about all that stuff and my running is coming along just fine.  If you get caught up in all the data collecting (or restricting) and "science", you can get overwhelmed.  I did.  I just try to be consistent with my running, and with each race, my runs are a little better.  No magic bullets, you just have to put the time and miles in, then things start coming together on their own.

HR training is great, and I'm thinking about starting a plan using it, but when you're just starting out, I don't think it helps much.

For running:  Mostly easy, sometimes hard.

Listen to your body.  Recovery is important.

An IM is doable with a year's worth of training.  I wish I'd of done my first IM in my second season when I was 35 rather than waiting till I was 44.

My signature line.

Basically if you are going to commit to a sport like this, there will be times where you are tired, maybe uncomfortable, etc... but it's part of the journey and completely worth it in the end (IMO).

I talk to a lot of triathletes who almost seem surprised, and then get discouraged, when they find themselves fatigued or tired or exhausted. Expect it, and embrace it 

Remember that it's a hobby.  When I focus too much on results and improvement I get super burned out.  Working with a coach was even worse.  When I remember how much I love to ride my bike, what it's like to run in the early morning, and how fun/relaxing swimming can be I do much, much better.

Also, I forgot...pay more attention when you're riding easy.  That's when I get into trouble.

Block out any feelings as you encounter things that are hard for you. Simply note where you stand at the beginning of the process, apply and carry out a plan, then look back. That's when you let the emotion in, because it's pure joy.

You don't need much equipment or to spend much money to participate. Running shoes, a bike, something to wear, and some decent fitting goggles. Maybe a cheapo wetsuit. This applies to beginners AND experienced triathletes.

Forget about HRMs, powermeters, fancy wheels, transition bags, carbon whatever, Newtons, arm warmers, arm coolers, disc covers, powercranks, aerohelmets, personalized nutrition, blah blah blah. This industry has a way of making you feel like you need a ton of stuff to fit in, to be competitive. You don't.  

ETA: One other thing: Join a tri club. Between the comraderie, motivation to work harder, access to cheap used equipment, amount you'll learn from folks that have been there - done that, it's the best investment a beginner can make IMHO.

Your time on the bike will help your run, but your run won't help you on the bike!  Don't sweat some missed workouts, life happens... Just don't miss (or re-schedule) your long runs, long rides, etc.  Don't spend too much time in the discipline you are best in (if you are mostly a swimmer, spend more time on the bike and run).  Enjoy the training AND the events.

Life is too short to run in bad socks.

Get a woman to body mark you at races. They are generally neater.

Don't even think about talking about your training, races or pains to people who are not triathletes.

Your race number will probably never look good on your bike.

Don't buy new clothes in the first three months of training. They won't fit in a month.

That EVERYONE is as nervous as you are on race day - whether you've done 50 or 0 races...

Unfortunately pickle buckets are frowned upon, like the socks & sandals of the tri world.

That, yes, people pee in their wetsuits. Don't drink the lake water...

And keep that in mind when you rent a wetsuit...