Question - do you have the iphone? iphone 4S? Because if you look at Wahoo Fitness, you can add their HR and the Runmeter app. I use this and really recommend it. If you have the iphone 4s, you can get the low power bluetooth wireless HR belt for about 70 bucks and it integrates seamlessly with this and many other fitness apps,
I used to run by feel, but this year, after a comprehensive treadmill test and getting an iphone 4s, I've gotten really into being more aware of the data, dumping maps and times into my calendar, and re-focusing training. If you have an older iphone, you can use a ANT enabled heart rate monitor. I also recommend getting some accurate data on your max heart rate to set up your training zones as well.
You could, of course, buy a Garmin or Polar, but having heart rate monitoring integrated into a good running app is more convenient, an integrated solution that allows you to get prompts in your headphones, graphs of your HR data with all the other data - pace, splits, maps, etc. so all your running information is in one place, with a good UI. I really like these two newish companies, Wahoo and Abvio
All of this, of course, depends on you having the smartphone to begin with. If you are an android user, look at Runkeeper but for the iPhone, i use
[Runmeter][1]
[Wahoo Fitness HR][2]
[1]: http://www.abvio.com/runmeter/
[2]: http://www.wahoofitness.com/Products/Wahoo-Fitness-Wahoo-Fitness-Blue-HR-Heart-Rate-Strap.asp