Here's what works for me
If you don't have a taste for techno, develop one. The high BPM (135+) songs not only give you a beat to match; they also give you a rate of momentum to match so you can measure when you're starting to fall off.
Try to sync into your own little world. One of the greatest barriers (at least for me) of working out is the social awkwardness of doing it alone. The better you're able to tune out the people around you the less anxiety you'll feel about doing activities by yourself.
I use two different (and both very high quality) podcasts for workout music. Podrunner and Beatport Burners.
Learn how to make snap decisions
It may seem pretty rudimentary but the ability to make very quick precise decisions is a good skill to have when you need to step out of your comfort zone.
Here's one strategy I've used to build this skill in the past. When you wake up in the morning (for work or school) instead of milling around in bed, immediately jump out of bed as if your house is on fire.
The more time you spend dwelling on a decision the harder it is to make.
Another would be to literally split the decision into two different options and flip a coin to decide. Instead of a lazy/active decision make it an active/active decision. For instance, instead of deciding whether or not to go workout, make it 'workout at the gym' or 'go for a run'. That way you have no path to laziness. The coin is only used as a crutch to get over the anxiety barrier involved in decision making.
IMHO, contrary to popular belief, indecisiveness has very little to do with laziness and very much to do with anxiety. Many people are lazy simply because they're too afraid to break out of their comfort zone. The reward (benefit of getting out) has to be greater than the risk (fear/anxiety involved in the unknown of trying something new).
Shaming yourself into believing that your just a 'lazy person' only makes it harder to become motivated because now you have to overcome both the anxiety of trying something new and the insecurity created by your self-image as a 'lazy person' (ie the risk increases).
Imagine the different ways you can critique a person. First you can attack a person's actions (why don't you get off your butt and get outside), attack their traits (why are you being lazy), or attack their person (you are a lazy person). The first two are easy to change, the third... not so much because you're describing the essence of what that person represents, not their actions. Take this idea into consideration when you critique yourself. People have the singular ability to destroy their own self-esteem without any external interaction because of this. Learning how to separate the 'who I am' from the 'what I do sometimes' is an important skill to learn to prevent from any unnecessary confidence speed bumps.
Adding an imaginary sense of danger is a means to increase the reward of getting out (escape from danger) whereas flipping a coin leaves the decision up to a higher power (luck, god, ect). I could probably write a dissertation about how religion relates to the higher power aspect of decision making but I'll spare you.
A mental barrier is not always strictly mental. Sometimes, a lack of energy/motivation can be due to your diet/health. For instance: if you're not getting enough sleep your hormones are likely not balanced properly; if you overtrain, your body hasn't had enough time to properly recover yet; if you don't have enough energy you will feel like you don't have enough energy.
The solutions to these issues are simple: get enough rest; plan enough recovery time in between workouts (if you start to feel a decrease of energy); and eat enough (carb load the night/morning before a day of high energy activity) and be sure to eat a lot of protein right after an activity (protein uptake is greatest <45min after exercise) for a better recovery.
I have dealt with and continue to deal with these issues on a daily basis. I could probably be considered a high anxiety personality type (even though my general demeanor is extremely relaxed) because it takes an extra conscious push for me to get involved in a lot of the activities I do.
Fortunately, I've found that the more I get involved in, the easier it is to see the reward instead of the risk when it comes time to make those snap decisions.
Update: @Ivo Flipse just recently posted How Can We Get Willpower Back Once It Has Been Depleted. It's a good read and definitely related to the Op's question.