As an EA to senior management, how do you plan or prioritise your day to day work to impress your employer, and efficiently manage your workload?

No doubt, It is really a challenging role. The most important thing is to organise the normal day to day work in order.

Keep smiling and be positive, offer support when you are free to help other colleagues. Learn other task and hone your skills of different dept/areas. Maintain discipline and harmonious working environment.

Being small company, you could arrange to collect the birthdates of employees and organise the small get togather to celebrate employees birthdays. 

Yes, It is a very challenging job, But be quit give priority to all direct linked jobs to the BOSS, then also to the colleagues with open heart and smiley face.

I have a several "regular" duties I do in same time all the time (like breastfeed - MUST not skip it) e.g. , follow up of contracts to be signed I do every morning after office coffee, workers in/out report I always do at15.30, after that I am managing vehicle schedule for next day, and few other duties during my working hours also have fixed time.  In between I always have time to make my priorities for various duties or to help other employees, or to follow up certain things that require more time, etc.

Before you end a day, check out the things you have accomplished from the past and see where you got more nods from your employer. Fill your next day first hour rapport to that, and the next hour, and the next...till voila! you just saw the day is over and you are still there wanting to do more extra work! And yes, employer will see you so energetic and loving it at work, and employer will be more than satisfied to (give you a little more than the over worked load, ah hahaha!) praise you for a job well done. Be patient in doing so cause the first few days might seem usual work days to your employer, until they really turn their heads, and stand closer to you than the last days they graced your cubicle, or your office, or your space, how big the space you work in maybe. 

Then again, test the waters of every day, because some days are diamond, and some days are stone for people (sounds like John Denver, uh huh?). Meaning, while we, the employees, know our employer's daily "oh-hi-there-sweetie-how-are-you-have-a-nice-day-ahead!" attitude towards the employees, and we comfortably   work our day throughout to that scenario, there are times we literally see there is a storm ahead--through their ashen faces. With that, let us go back to those times when we were able to pacify the storms, have a quick two minute mind replay of how things may eventually work out if you take the plunge... and just do the possible-empathize with them, professionaly. The how depends on how far we know the character of our employer and how best we see we can handle the situation. Pass a work when it's not yet due. Bring coffee during 15-minute tea breaks (coffee on tea, a hahaha!), or walk with them on the way to lunch...or so. Few little good things will count much to some employers, and rewards come when you least expect them. 

Above all, do your work before time, on time, before it's due, and   work happily happy as you want life to be.

cheers!

Basically, the challenges arise from the lack of proper planning and the lack of management of inputs and processes. Well, with proper planning, the amount of challenges can be increased, but it cannot be eliminated forever, that's why I am talking about implementation methods. Dealing with procedural risks

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TThis person should have a pleasant personality with efficiency and strong memories. Can spend a good tea time with colleagues but have to make boundaries and careful to discuss such sensitive matters and confidentiality is remains his or her priority.

Discipline , clear communication, rapid actions are the key to over come challenges. 

Think ahead of time and analse. 

Arranging the appointments based on priorities, try to have important and decision making requirements in the first half of the day, have routine meetings, call ons in the second half.

Organise the normal day to day work in order (Work Plan).

Managing your time, being proactive.

Offer support when you are free to help other colleagues.