In general, par is how you set graphical parameters for base graphics. Some graphical parameters can be set as arguments to plot. You can adjust axis spacing with mgp. Here are some simple examples:
# default values of mgp
plot(1:10, 1:10, mgp = c(3, 1, 0))
# move axis titles closer
plot(1:10, 1:10, mgp = c(2, 1, 0))
# move axis labels closer
plot(1:10, 1:10, mgp = c(2, 0.5, 0))
# move axis lines away from plot
plot(1:10, 1:10, mgp = c(3, 1, 0.5))