We are supposed to lean while drinking the 4 cups and eating the matza at the Seders, but I find it hard to do that while sitting in an armless dining room chair. Likewise, I have not been able to secure a pillow on such a chair. How do others deal with this little practical challenge? Any helpful tips would be appreciated. Note: there is another question Is there an ideal way to recline on Pesach? that asks what the ideal leaning posture is or might be. The current question just asks for practical approaches to leaning while seated around a dining room table.
I've turned the chair sideways (i.e., the chair back is perpendicular to the table), draped a pillow over the back and leaned my hand over the pillow and the back. Don't use a high-back chair for this. A short chair or folding chair will work fine for this purpose. Another easy solution is just lean your elbow on the table.
I have seen a number of ravs I have been with do just this.
Another simple solution (perhaps) is just put an empty chair to the left of yours and pile some soft books and at least one pillow on top of the pile. I know - it does mean an extra seat at the table. Hey! My adage is that there's always room for one more "dummy" :-)
The simplest thing that I have done in this situation is to pull over a folding chair, and set it perpendicular to the left of my chair. Then I use the back of that chair as my armrest. If you have space for it, this is probably your best option.
As a backup, I once turned to my right, so that the table was on my left, and then used the table to lean on.
At most seders I've been to, people are packed tightly-enough around the table that we end up kind of leaning into each other's spaces. Since we're all doing it, nobody is inconvenienced.
Sometimes, like @msh210, I've turned in my seat so that I can lean against the back of the chair.
עוד כתבו הפוסקים דאם סומך עצמו על ברכי חבירו גם זה מיקרי הסיבה ע"פ הדחק
The halachic decisors have written further that one can lean on the lap of his friend, and this is also considered to be "leaning" in a difficult situation.
Mishna Berurah 472:8, free translation.