#1 Practice, practice, practice.
#2 Realize that shot selection
is as important as shot execution
#3 Recognize that neither of those shots has to be perfect to be effective.
Now for some real tips (I hope):
On the slice, take your time, be patient with making/approaching the shot (unless you're doing it from a defensive stand-point...at which point, you'll probably do it out of luck/happenstance). Taking a deep breath as I strike the ball seems to help. I also try to "lean" or "lead" into the shot with my shoulder, rather than trying to develop power/depth from my knees/legs.
Then, of course, there are the obvious ones: work the stroke high-to-low and keep your eye on the ball all the way to the racket. Doesn't have to be extreme high to extreme low but try to let your body
mechanics effect the shot rather than trying to do it with just you arm or wrist (is that even possible for slice?). The harder the ball is coming at you, the less high-to-low you need to be. Use their power against them.
Now on the lob, unless you have a really tall opponent or one with a potent AND consistent overhead...you shouldn't be afraid to use this shot more often. Start with trying to hit them flat (then add a topspin component) and focus on getting the ball to drop at least as far back as the service line although I believe it's better to err on the "long" side rather than the short. Opponents will often make flinging/flaying attempts at it (especially if they're have to run back
for it) and if it does fall out...you've made them run for it...and perhaps can capitalize on their poor fitness on the next point.
Ooops: forgot to add: Be sure to practice your lob during the warm-up period. It'll let you "dial in" the range, so to speak...and let your opponent know it's a component of your game. I find it particularly useful, if not necessarily "nice" to throw one in when they're at the net...expecting a shot to volley. Gives me a chance to see how they react (footwork...and an unexpected OH).
for trying to improve some underrated and underused shots (IMHO