I rely on my ~/.bash_history extensively to avoid retyping things, both short-term and long term (when I want to look up something I typed yesterday or last week).

First, an obvious necessity is

shopt -s histappend

in ~/.bashrc, so that bash just keeps on adding to the end of the ~/.bash_history file rather than obliterate the file at regular intervals.

By default, bash reads from your ~/.bash_history once when you log in to a terminal, and it updates ~/.bash_history once when you log out. I dislike both of these default behaviors.

Writing out to ~/.bash_history only when you log out is no good, because what if your terminal dies unexpectedly? You'll lose the history of any commands you typed there. This can happen if you kill a bash process, or if the machine powers off unexpectedly, or who knows what. So, to manually update your ~/.bash_history immediatety, at any time you can use this:

history -a

which tells bash to append to ~/.bash_history any commands in the current terminal that aren't already in there.

The second behavior of bash I dislike is that it reads from the history file only at login, mostly because I always have many terminals open at once. I run a command from one terminal, then run a different command from another, switch back and forth etc. These terminals may be appending all kinds of nice new lines to the end of my ~/.bash_history, but because bash never reads the history file except at login, they're never accessible except in the terminal where the command was originally run. Luckily you can force bash to re-read your history file via:

history -n

which tells bash to read any new lines that have appeared in ~/.bash_history since the last time it looked.

Now, say I type a really long command, e.g. a huge long Perl one-liner, in one terminal, and I want to then access and run that command in another terminal via bash history. With bash's default behavior, I can't do it; each terminal has its own history. What I could do is type history -a in the first terminal, and history -n in the second terminal; then it'd work wonderfully.

I find the thought of having to do that all the time tedious, though. Better would be for bash to do it for me every time I run a command. You can accomplish this by putting something like this in ~/.bashrc:

export PROMPT_COMMAND="history -n; history -a"

PROMPT_COMMAND lets you specify a command that bash will run every time it shows you a fresh command prompt, i.e. every time you run a command and the command finishes. So the above tells bash to read any new lines that have appeared in ~/.bash_history since the last time it read it, and then append the last-run command from this terminal to ~/.bash_history, every time you run a command.

So now, if you type a command in one terminal, and want to access it via the history of another terminal, run a command in the other terminal (or just hit Enter) to trigger PROMPT_COMMAND, and then your history will be nicely up-to-date and synchronized with any other terminals you have open. Almost certainly, you'll never notice the tiny bit of overhead caused by bash constantly reading and writing to ~/.bash_history.

See man bash for more info on the history builtin.