What can be done when one is having difficulties establishing a regular practice? I just can't manage to maintain a regular practice, not even a short one. From a buddhist point of view, what can be done? 

I went on retreats, I sat a few days in a row at home doing meditation, I tried to do small meditations, but I just can't sustain a practice.
This happens when you have a mistaken idea that practice is something separate from your normal life. But meditation is only 1/3rd of the practice, the top of the wedding cake (shila-prajna-samadhi). Most of the practice happens during the day, in what some of us Buddhists semi-jokingly call "post-meditation".

I think meditation looks important to Westerners because it seems to follow familiar functional pattern: do something and get the result. "Leave the rest of your Buddhist B.S. superstitions to the uneducated ancients. I will focus on THE practice." But Buddhism does not work this way. Without right context meditation is useless. 

What is the context in Buddhism? The context is removing obstacles to enlightenment, from coarse obstacles, to middling obstacles, to subtle obstacles. You can't remove subtle obstacles until you remove middling obstacles. You can't remove middling obstacles until you remove coarse obstacles. Meditation is a tool that is used for working with subtle obstacles.

What are coarse obstacles? They are coarse biases, coarse illusions, coarse preconceptions, coarse hangups. How can you identify them? By their effects -- production of strong emotions during the day.

Every time you feel strong emotion (e.g. longing, irritation, frustration, resentment, contempt, insult, disgust, anxiety, aversion, strong timidity) - or about to feel strong emotion - therein is attachment (=bias, illusion, preconception, hangup). That's the moment when you should switch your context from whatever it is that you are grasping at, and pause and look at your attachment. At such moments it is right there, like an elephant in the room. What's interesting about attachment, subjectively it always looks legit, it always seems right, appropriate, justified. But that's the way it tricks the rational mind. It may even be attachment to humbleness, attachment to open-mindedness, attachment to kindness, attachment to fairness - any of the good things. But when it becomes all-important it turns into a demon and starts generating judgement, aversion, conflicts, blinding emotions, and bad karma. So as soon as you see the emotion arising - you should step back and identify the attachment / preconception. Then you should let it go while focusing on your recommended anchor:


if you are very mental, focus on your body - esp. lower abdomen and feet. Do you feel any tightness or burning or blockage? Do you feel other parts of your body or are they numb?,
if you are very emotional, focus on your breathing - is it fluid or jerky or blocked?,
if you take real-life situations very seriously, imagine the time of your death,
if you are an earthly person, remember that you exist in the infinite Universe with its worlds, which existed for billions of years and will exist for billions of years into the future,
if you often feel helpless or frustrated getting your point across, remember behavior of Buddha or your teacher.
in any case, let go of your attachment and change your point of view to either very low level (physiology of your psychosomatic expressions and the mechanics of the imprint-based associative cycle) or very high level (the perspective of "God").


When you practice like this you can't forget to practice, there is no problem "establishing" practice - every time there is an obstacle in your life it comes as a reminder to practice!

In the same vein, when you notice yourself getting too happy, too self-righteous, enjoying yourself too much, being too puffed up, doing too well - you should pause and check if there is an attachment at play. When you're doing this right, there should be a feeling of balance, some feeling of ambiguity or open-endedness. But with this I am getting into advanced territory... Anyway.

This is what the practice should be on initial stages, this was my main practice for many years - this and the overcoming of spiritual materialism - before I was even introduced to Buddhist philosophy of anatta and shunyata - and only after I mastered those two conceptually, was I introduced to (shikantaza) meditation.

So yeah like I said, to any Westerner jumping straight into meditation I recommend at least five years of preliminary "shila" practice working with attachments (coarse obstacles), then five years of "prajna" practice studying -- and applying! -- anatta and shunyata to remove middling obstacles, and only then the practice of "samadhi" aka meditation.