Systemd has a neat little feature where a daemon can turn on whenever a connection is made to it. Works with Unix and network sockets. More details here. Essentially you create a socket file alongside your service file. The advantage is that you don't need to start your services in a specific order.
Flush old logs in journalctl
By date or by size:
sudo journalctl --vacuum-time=2d
sudo journalctl --vacuum-size=500M
For a specific service:
journalctl -u httpd -f
Store logs on disk
On CentOS 7, you have to enable the persistent storage of log messages:
# mkdir /var/log/journal
# systemd-tmpfiles --create --prefix /var/log/journal
# systemctl restart systemd-journald
Otherwise, the journal log messages are not retained between boots. This is the default on Fedora 19+.