As an empath, I cannot enter a crowded space without creating a bubble of protection around myself. I know I will feel everything others are experiencing on a physical/mental/emotional level. If I forget to protect myself, I return home exhausted and drained, barely able to walk up the driveway
to my home. I urge you to try this exercise...it really does work!
Some examples of how you'll know if you need to protect yourself:
Are you feeling low on energy, as if even the slightest activity takes all your resources and focus? When you go shopping, or go to a crowded public place, do you get strange aches and pains or feel sapped by the time you've finished, even though you were only there for a brief time? Do you leave home bouncing off the walls, but after ten minutes at work, feel as if you've been run over by a steam roller? Have you ever visited someone and left depressed and lethargic, even though the conversation was lively and happy?
Sometimes we need to look at the people around us and the effect they are having on our energetic field. True there are some who are 'energy vampires', who recharge just by being around us, but they're not the only ones who deplete our resources.
Be aware of the people around us, the ones we 'hang out' with, our workmates and even passengers travelling on the same bus. All too often we blithely wander through our day, not realising that 'pieces' of us are being sucked dry by the negative energy of others.
It's important to 'shut ourselves off' which can be as easy as imagining a bubble of protective light around our energetic field. It doesn't require meditation and rituals or anything fancy and mystical. All you need is your focussed intent. 
Before you leave the house, imagine a bubble of energy forming around your energetic field. If you have trouble imagining or visualising one, just say the words out loud, or draw a picture if it helps. Intent really is everything! It only takes a fraction of time and the benefits are huge.
There's no particular colour you need to use, in fact your favourite colour, or the colour you feel you need around you that day can be perfect. The only colours you never ever use is black or it's variations of grey. Some people use the colour of a specific chakra that needs attention and others prefer a clear bubble. If you want to have a rainbow bubble, then go ahead - it's your bubble, you get to choose - you can even use it to accessorise your outfit if you like!
You can still do it later, if you're out and you forgot. Don't panic if you're halfway through grocery shopping when you remember or at lunchbreak. Half a day is better than no day!
If you have to, leave a reminder somewhere to jog your memory. I have a friend who has a piece of paper in the colour of her choice on her dashboard that reminds her to create a bubble before she starts out. There's no writing on it, she doesn't have to worry about people asking her why she has a 'Remember Your Bubble' sign in her car, and if anyone does ask about that coloured bit of paper next to the speedometer, I'm sure she could think of a feasible story anyway!
So, next time you go out, make a conscious action to close yourself off to protect your energy and vitality. I'll bet you notice the difference!
with love, respect and integrity
Cherie xx
PS. You can also create a bubble of protection for someone else, like your children, a sick friend or elderly family member.